[AMDGCN] Use ZExt when handling indices in insertment element (#85718)

When i1 true is used as an index, SExt extends it to i32 -1. This would
cause BitVector to overflow.
The language manual have specified that the index shall be treated as an
unsigned number, this patch fixes that.
(https://llvm.org/docs/LangRef.html#insertelement-instruction

)

This patch fixes #85717

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Signed-off-by: Peter Rong <PeterRong96@gmail.com>