Add intrinsic forms of mmx<->sse conversions. Notes:

Omission of memory form of PI2PD is intentional; this
does not use an MMX register and does not put the chip
into MMX mode (PI2PS, oddly enough, does).
Operands of PI2PS follow the gcc builtin, not Intel.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@113388 91177308-0d34-0410-b5e6-96231b3b80d8