[clang] [llvm] [X86] Support reserving EDI and ESI on x86-32 (PR #186123)

[Eli Friedman via cfe-commits]

https://github.com/efriedma-quic commented:

I'm not really opposed in general, I guess? It's just a matter of whether it's actually practical for the specific registers in question.

ESI in particular has a problem this patch doesn't address: we use it as the base register, to refer to the stack in certain functions.  See X86RegisterInfo::X86RegisterInfo .

https://github.com/llvm/llvm-project/pull/186123