strided-stores-vectorized.ll crashes for RV32 without fixing the
relevant logic in vectorizeTree, because the argument can't be
represented as a 32-bit unsigned value:
```
llvm::APInt::APInt(unsigned int, uint64_t, bool, bool): Assertion `llvm::isUIntN(BitWidth, val) && "Value is not an N-bit unsigned value"' failed.
```
It is intended to be signed, so we simply use ConstantInt::getSigned
instead. This fixes other stride-related instances in the file as well.
For further context, this change is part of unblocking rv32gcv
llvm-test-suite in CI.
3ae71d30be