[llvm] [WebAssembly][FastISel] Fix sext i1 to i64 with +sign-ext (#213734) (PR #214007)

[Gauarv Chaudhary via llvm-commits]

ANAMASGARD wrote:

Sir @QuantumSegfault I have  restored the upstream Result/Reg reuse pattern for i8/i16 in `signExtend()`.
The CI failure in `fast-isel-atomic-fold.ll` was caused by a virtual-register push-order change (Tmp vs Result), not a semantic codegen change. The test passes unchanged locally after this follow-up commit.

Please re run the CI test so that we can see the result , Thanks 

https://github.com/llvm/llvm-project/pull/214007