GlobalISel: Add target pre-isel instructions

GlobalISel: Add target pre-isel instructions

Allows targets to introduce regbankselectable
pseudo-instructions. Currently the closet feature to this is an
intrinsic. However this requires creating a public intrinsic
declaration. This litters the public intrinsic namespace with
operations we don't necessarily want to expose to IR producers, and
would rather leave as private to the backend.

Use a new instruction bit. A previous attempt tried to keep using enum
value ranges, but it turned into a mess.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@373937 91177308-0d34-0410-b5e6-96231b3b80d8