Change TargetLowering::getRegClassFor to take an MVT, instead of EVT.

Accordingly, add helper funtions getSimpleValueType (in parallel to
getValueType) in SDValue, SDNode, and TargetLowering.

This is the first, in a series of patches.

This is the second attempt. In the first attempt (r169837), a few
getSimpleVT() were hoisted too far, detected by bootstrap failures.

llvm-svn: 170104