This fixes an internal crash caused by D106555 on the 32-bit AIX target for v16i8/v8i16 splat load lowering.
(INSERT_SUBREG (i64 (IMPLICIT_DEF)), (LHZX ForceXForm:$A), sub_32) causes an assertion later:
Assertion failed: RC && "This value type is not natively supported!", file /home/czhengsz/llvm/dev/llvm-project/llvm/include/llvm/CodeGen/TargetLowering.h, line 863, virtual const llvm::TargetRegisterClass *llvm::TargetLoweringBase::getRegClassFor(llvm::MVT, bool) const()
@nemanjai suggested using MTVSRWZ instead of MTVSRD in the post-commit review of D106555, this can also avoid the INSERT_SUBREG, so it can solve the crash on 32-bit AIX.