GlobalISel: add a G_PHI instruction to give phis a type.

They're another source of generic vregs, which are going to need a type on the
definition when we remove the register width from MachineRegisterInfo.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@280412 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed