[GlobalISel] Import patterns containing SUBREG_TO_REG

Reuse the logic for INSERT_SUBREG to also import SUBREG_TO_REG patterns.

- Split `inferSuperRegisterClass` into two functions, one which tries to use
  an existing TreePatternNode (`inferSuperRegisterClassForNode`), and one that
  doesn't. SUBREG_TO_REG doesn't have a node to leverage, which is the cause
  for the split.

- Rename GlobalISelEmitterInsertSubreg.td to GlobalISelEmitterSubreg.td and
  update it.

- Update impacted tests in the AArch64 and X86 backends.

This is kind of a hit/miss for code size improvements/regressions. E.g. in
add-ext.ll, we now get some identity copies. This isn't really anything the
importer can handle, since it's caused by a later pass introducing the copy for
the sake of correctness.

Differential Revision: https://reviews.llvm.org/D66769

llvm-svn: 370254