[InstCombine] Missing zext(shl(trunc)) fold #61650
Description
From https://reviews.llvm.org/D146637:
define i32 @src(i32 %a) {
%trunc = trunc i32 %a to i16
%shl = shl i16 %trunc, 8
%ext = zext i16 %shl to i32
ret i32 %ext
}
define i32 @tgt(i32 %a) {
%shl = shl i32 %a, 8
%and = and i32 %shl, 65280
ret i32 %and
}