[AMDGPU] Uses SExt on indices in `isInterestingPHIIncomingValue`

[DataCorrupted]

Reproduction

https://godbolt.org/z/P97TT7fEs

Cause

When i1 true is used as an index, SExt extends it to i32 -1. This would cause BitVector to overflow.
This can be a quick fix by using ZExt over SExt, as we have specified in the language manual that the index shall be treated as an unsigned number. (https://llvm.org/docs/LangRef.html#insertelement-instruction)

[llvmbot]

@llvm/issue-subscribers-backend-amdgpu

Author: Peter Rong (DataCorrupted)

## Reproduction

https://godbolt.org/z/P97TT7fEs

Cause

When i1 true is used as an index, SExt extends it to i32 -1. This would cause BitVector to overflow.
This can be a quick fix by using ZExt over SExt, as we have specified in the language manual that the index shall be treated as an unsigned number. (https://llvm.org/docs/LangRef.html#insertelement-instruction)