Reuse the logic for INSERT_SUBREG to also import SUBREG_TO_REG patterns. There are some intrinsic + load/store patterns in AArch64 that use this.
- 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.