[GlobalOpt] Don't remove inalloca from varargs functions

Varargs and inalloca have a weird interaction where varargs are actually
passed via the inalloca alloca. Removing inalloca breaks the varargs
because they're still not passed as separate arguments.

Fixes #58718

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D137182

(cherry picked from commit 8c49b01a)