[llvm] [win][arm64ec] More fixes for building and testing Arm64EC Windows (PR #151409)

[Daniel Paoliello via llvm-commits]

dpaoliello wrote:

> LLVM Buildbot has detected a new failure on builder `mlir-nvidia-gcc7` running on `mlir-nvidia` while building `llvm` at step 7 "test-build-check-mlir-build-only-check-mlir".
> 
> Full details are available at: https://lab.llvm.org/buildbot/#/builders/116/builds/16594
> 
> Here is the relevant piece of the build log for the reference

Seems unrelated. This change only affected codegen for Arm64EC and modified two specific tests. 

https://github.com/llvm/llvm-project/pull/151409