[SelectionDAG] Ignore LegalTypes parameter in TargetLoweringBase::getShiftAmountTy. (#97645)

When this flag was false, `getShiftAmountTy` would return `PointerTy`
instead of the target's preferred shift amount type for scalar shifts.

This used to be needed when the target's preferred type wasn't large
enough to support the shift amount needed for an illegal type. For
example, any scalar type larger than i256 on X86 since X86's preferred
shift amount type is i8.

For a while now, we've had code that uses `MVT::i32` if `LegalTypes` is
true, but the target's preferred type is too small. This fixed a
repeated cause of crashes where the `LegalTypes` flag wasn't set to
false when illegal types could be present.

This has made it unnecessary to set the `LegalTypes` flag correctly, and
as a result more and more places don't. So I think its time for this
flag to go away.

This first patch just disconnects the flag. The interface and all
callers will be cleaned up in follow up patches.

The X86 test change is because we now have the same shift type for both
shifts in a (srl (sub C, (shl X, 32), 32) sequence. This makes the shift
amounts appear equal in value and type which is needed to enable a
combine.

Author: topperc

llvm/lib/CodeGen/TargetLoweringBase.cpp

@@ -1057,8 +1057,7 @@ EVT TargetLoweringBase::getShiftAmountTy(EVT LHSTy, const DataLayout &DL,
   assert(LHSTy.isInteger() && "Shift amount is not an integer type!");
   if (LHSTy.isVector())
     return LHSTy;
-  MVT ShiftVT =
-      LegalTypes ? getScalarShiftAmountTy(DL, LHSTy) : getPointerTy(DL);
+  MVT ShiftVT = getScalarShiftAmountTy(DL, LHSTy);
   // If any possible shift value won't fit in the prefered type, just use
   // something safe. Assume it will be legalized when the shift is expanded.
   if (ShiftVT.getSizeInBits() < Log2_32_Ceil(LHSTy.getSizeInBits()))

llvm/test/CodeGen/X86/shift-combine.ll

@@ -444,12 +444,10 @@ define i64 @ashr_add_neg_shl_i32(i64 %r) nounwind {
 define i64 @ashr_add_neg_shl_i8(i64 %r) nounwind {
 ; X86-LABEL: ashr_add_neg_shl_i8:
 ; X86:       # %bb.0:
-; X86-NEXT:    movl {{[0-9]+}}(%esp), %eax
-; X86-NEXT:    shll $24, %eax
-; X86-NEXT:    movl $33554432, %edx # imm = 0x2000000
-; X86-NEXT:    subl %eax, %edx
-; X86-NEXT:    movl %edx, %eax
-; X86-NEXT:    sarl $24, %eax
+; X86-NEXT:    movb $2, %al
+; X86-NEXT:    subb {{[0-9]+}}(%esp), %al
+; X86-NEXT:    movsbl %al, %eax
+; X86-NEXT:    movl %eax, %edx
 ; X86-NEXT:    sarl $31, %edx
 ; X86-NEXT:    retl
 ;