[llvm] Reland "RegisterCoalescer: Add implicit-def of super register when coalescing SUBREG_TO_REG" (PR #168353)

[Nathan Corbyn via llvm-commits]

cofibrant wrote:

We've have failures on https://green.lab.llvm.org/job/lnt-ctmark-aarch64-O3-flto/ since this relanded, with an assert tripping in `RegisterCoalescer.cpp`.

https://github.com/llvm/llvm-project/pull/168353