Add isExtractSubreg property.

This patch adds a new property: isExtractSubreg and the related target hooks:
TargetIntrInfo::getExtractSubregInputs and
TargetInstrInfo::getExtractSubregLikeInputs to specify that a target specific
instruction is a (kind of) EXTRACT_SUBREG.

The approach is similar to r215394.

<rdar://problem/12702965>