[flang]: This is to fix the HLFIR path for PPC Vector type intrinsics. (#66547)

PowerPC Vector type intrinsics currently crashes with
`-flang-experimental-hlfir` is specified.

This patch is to fix the HLFIR path for PowerPC Vector type intrinsics.

The patch:
1. Added the `flang-experimental-hlfir` option to all PowerPC vector intrinsic testing.
2. Removed the FIR/MLIR testing to reduce the maintenance cost.
3. Fixed a few verification IR for some non-functional changes in LLVM IR in HLFIR path.