[X86][AVX] Use extract_subvector to reduce vector op widths (PR36761)

We have a number of cases where we fail to reduce vector op widths, performing the op in a larger vector and then extracting a subvector. This is often because by default it would create illegal types.

This peephole patch attempts to handle a few common cases detailed in PR36761, which typically involved extension+conversion to vX2f64 types.

Differential Revision: https://reviews.llvm.org/D49556

llvm-svn: 337500