[X86] X86InstCombineIntrinsic.cpp - silence clang-tidy warnings about incorrect uses of auto. NFCI.

We were using auto instead of auto* in a number of places which failed the llvm-qualified-auto check.

Additionally we were using auto in some places where the type wasn't immediately obvious - the style guide rule of thumb is only to use auto from casts etc. where the type is already explicitly stated.