Backport some LLVM compile-time improvements
Backport some LLVM compile-time improvements

This backports two compile-time improvements from the LLVM 13
branch, which reduce the number of alias analysis queries.

aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust