[X86] When doing callee save/restore for k-registers make sure we don't use KMOVQ...

[X86] When doing callee save/restore for k-registers make sure we don't use KMOVQ on non-BWI targets

If we are saving/restoring k-registers, the default behavior of getMinimalRegisterClass will find the VK64 class with a spill size of 64 bits. This will cause the KMOVQ opcode to be used for save/restore. If we don't have have BWI instructions we need to constrain the class returned to give us VK16 with a 16-bit spill size. We can do this by passing the either v16i1 or v64i1 into getMinimalRegisterClass.

Also add asserts to make sure BWI is enabled anytime we use KMOVD/KMOVQ. These are what caught this bug.

Fixes PR36256

Differential Revision: https://reviews.llvm.org/D42989

llvm-svn: 324533