[TBAA] Use !tbaa for first accessed field if it is an exact match in offset and size. (#81313)

Motivation for this and follow-on patches is to improve codegen for
libc++, where using memcpy limits optimizations, like vectorization for
code iteration over std::vector<std::complex<float>>:
https://godbolt.org/z/f3vqYos3c

Depends on #81289.

PR: #81313

Author: fhahn

llvm/lib/Analysis/TypeBasedAliasAnalysis.cpp

@@ -821,7 +821,7 @@ MDNode *AAMDNodes::extendToTBAA(MDNode *MD, ssize_t Len) {
 AAMDNodes AAMDNodes::adjustForAccess(unsigned AccessSize) {
   AAMDNodes New = *this;
   MDNode *M = New.TBAAStruct;
-  if (M && M->getNumOperands() == 3 && M->getOperand(0) &&
+  if (M && M->getNumOperands() >= 3 && M->getOperand(0) &&
       mdconst::hasa<ConstantInt>(M->getOperand(0)) &&
       mdconst::extract<ConstantInt>(M->getOperand(0))->isZero() &&
       M->getOperand(1) && mdconst::hasa<ConstantInt>(M->getOperand(1)) &&

llvm/test/Transforms/SROA/tbaa-struct2.ll

@@ -11,11 +11,11 @@ declare double @subcall(double %g, i32 %m)
 define double @bar(ptr %wishart) {
 ; CHECK-LABEL: @bar(
 ; CHECK-NEXT:    [[TMP_SROA_3:%.*]] = alloca [4 x i8], align 4
-; CHECK-NEXT:    [[TMP_SROA_0_0_COPYLOAD:%.*]] = load double, ptr [[WISHART:%.*]], align 8, !tbaa.struct [[TBAA_STRUCT0:![0-9]+]]
+; CHECK-NEXT:    [[TMP_SROA_0_0_COPYLOAD:%.*]] = load double, ptr [[WISHART:%.*]], align 8, !tbaa [[TBAA0:![0-9]+]]
 ; CHECK-NEXT:    [[TMP_SROA_2_0_WISHART_SROA_IDX:%.*]] = getelementptr inbounds i8, ptr [[WISHART]], i64 8
-; CHECK-NEXT:    [[TMP_SROA_2_0_COPYLOAD:%.*]] = load i32, ptr [[TMP_SROA_2_0_WISHART_SROA_IDX]], align 8, !tbaa [[TBAA5:![0-9]+]]
+; CHECK-NEXT:    [[TMP_SROA_2_0_COPYLOAD:%.*]] = load i32, ptr [[TMP_SROA_2_0_WISHART_SROA_IDX]], align 8, !tbaa [[TBAA4:![0-9]+]]
 ; CHECK-NEXT:    [[TMP_SROA_3_0_WISHART_SROA_IDX:%.*]] = getelementptr inbounds i8, ptr [[WISHART]], i64 12
-; CHECK-NEXT:    call void @llvm.memcpy.p0.p0.i64(ptr align 4 [[TMP_SROA_3]], ptr align 4 [[TMP_SROA_3_0_WISHART_SROA_IDX]], i64 4, i1 false), !tbaa.struct [[TBAA_STRUCT7:![0-9]+]]
+; CHECK-NEXT:    call void @llvm.memcpy.p0.p0.i64(ptr align 4 [[TMP_SROA_3]], ptr align 4 [[TMP_SROA_3_0_WISHART_SROA_IDX]], i64 4, i1 false), !tbaa.struct [[TBAA_STRUCT6:![0-9]+]]
 ; CHECK-NEXT:    [[CALL:%.*]] = call double @subcall(double [[TMP_SROA_0_0_COPYLOAD]], i32 [[TMP_SROA_2_0_COPYLOAD]])
 ; CHECK-NEXT:    ret double [[CALL]]
 ;
@@ -38,14 +38,13 @@ define double @bar(ptr %wishart) {
 ;.
 ; CHECK: attributes #[[ATTR0:[0-9]+]] = { nocallback nofree nounwind willreturn memory(argmem: readwrite) }
 ;.
-; CHECK: [[TBAA_STRUCT0]] = !{i64 0, i64 8, [[META1:![0-9]+]], i64 8, i64 4, [[TBAA5]]}
-; CHECK: [[META1]] = !{[[META2:![0-9]+]], [[META2]], i64 0}
-; CHECK: [[META2]] = !{!"double", [[META3:![0-9]+]], i64 0}
-; CHECK: [[META3]] = !{!"omnipotent char", [[META4:![0-9]+]], i64 0}
-; CHECK: [[META4]] = !{!"Simple C++ TBAA"}
-; CHECK: [[TBAA5]] = !{[[META6:![0-9]+]], [[META6]], i64 0}
-; CHECK: [[META6]] = !{!"int", [[META3]], i64 0}
-; CHECK: [[TBAA_STRUCT7]] = !{}
+; CHECK: [[TBAA0]] = !{[[META1:![0-9]+]], [[META1]], i64 0}
+; CHECK: [[META1]] = !{!"double", [[META2:![0-9]+]], i64 0}
+; CHECK: [[META2]] = !{!"omnipotent char", [[META3:![0-9]+]], i64 0}
+; CHECK: [[META3]] = !{!"Simple C++ TBAA"}
+; CHECK: [[TBAA4]] = !{[[META5:![0-9]+]], [[META5]], i64 0}
+; CHECK: [[META5]] = !{!"int", [[META2]], i64 0}
+; CHECK: [[TBAA_STRUCT6]] = !{}
 ;.
 ;; NOTE: These prefixes are unused and the list is autogenerated. Do not add tests below this line:
 ; CHECK-MODIFY-CFG: {{.*}}