Eliminate a batch of uses of sub_ss and sub_sd in the X86 target.

These idempotent sub-register indices don't do anything --- They simply
map XMM registers to themselves.  They no longer affect register classes
either since the SubRegClasses field has been removed from Target.td.

This patch replaces XMM->XMM EXTRACT_SUBREG and INSERT_SUBREG patterns
with COPY_TO_REGCLASS patterns which simply become COPY instructions.

The number of IMPLICIT_DEF instructions before register allocation is
reduced, and that is the cause of the test case changes.

llvm-svn: 160816