EXTRACT_SUBREG coalescing support. The coalescer now treats EXTRACT_SUBREG like

(almost) a register copy. However, it always coalesced to the register of the
RHS (the super-register). All uses of the result of a EXTRACT_SUBREG are sub-
register uses which adds subtle complications to load folding, spiller rewrite,
etc.

llvm-svn: 42899