Add isInsertSubreg property.

This patch adds a new property: isInsertSubreg and the related target hooks:
TargetIntrInfo::getInsertSubregInputs and
TargetInstrInfo::getInsertSubregLikeInputs to specify that a target specific
instruction is a (kind of) INSERT_SUBREG.

The approach is similar to r215394.

<rdar://problem/12702965>