[PowerPC] Fix %llvm.ppc.altivec.vc* lowering

Summary:
r372285 changed LLVM to use a `TargetConstant` for parameters of intrinsics that are required to be immediates.

Since that commit, use of `%llvm.ppc.altivec.vc{fsx,fux,tsxs,tuxs}` intrinsics has not worked, and resulted in a `LLVM ERROR: Cannot select: intrinsic %llvm.ppc.altivec.vc*` error. The intrinsics' TableGen definitions matched on `imm` instead of `timm`.

This commit updates those definitions to use `timm`.

Fixes: https://llvm.org/PR44239

Reviewers: hfinkel, nemanjai, #powerpc, Jim

Reviewed By: Jim

Subscribers: qiucf, wuzish, Jim, hiraditya, kbarton, jsji, shchenz, llvm-commits

Tags: #llvm

Patched by vddvss (Colin Samples).

Differential Revision: https://reviews.llvm.org/D71138