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

[Nathan Corbyn via llvm-commits]

cofibrant wrote:

@sdesmalen-arm do you have an idea for a fix or are we looking at reverting again?

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