Implement XMM subregs.

Extracting the low element of a vector is now done with EXTRACT_SUBREG,
and the zero-extension performed by load movss is now modeled with
SUBREG_TO_REG, and so on.

Register-to-register movss and movsd are no longer considered copies;
they are two-address instructions which insert a scalar into a vector.

llvm-svn: 97354