[X86] Add patterns to turn an insert into lower subvector of a zero vector...

[X86] Add patterns to turn an insert into lower subvector of a zero vector into a move instruction which will implicitly zero the upper elements.

Ideally we'd be able to emit the SUBREG_TO_REG without the explicit register->register move, but we'd need to be sure the producing operation would select something that guaranteed the upper bits were already zeroed.

llvm-svn: 312450