[X86] Generate v2f64 X86ISD::UNPCKL/UNPCKH instead of X86ISD::MOVLHPS/MOVHLPS...

[X86] Generate v2f64 X86ISD::UNPCKL/UNPCKH instead of X86ISD::MOVLHPS/MOVHLPS for unary v2f64 {0,0} and {1,1} shuffles with SSE2.

I'm trying to restrict the MOVLHPS/MOVHLPS ISD nodes to SSE1 only. With SSE2 we can use unpcks. I believe this will allow some patterns to be cleaned up to require fewer bitcasts.

I've put in an odd isel hack to still select MOVHLPS instruction from the unpckh node to avoid changing tests and because movhlps is a shorter encoding. Ideally we'd do execution domain switching on this, but the operands are in the wrong order and are tied. We might be able to try a commute in the domain switching using custom code.

We already support domain switching for UNPCKLPD and MOVLHPS.

llvm-svn: 337348