Auto merge of #135274 - saethlin:array-repeats, r=compiler-errors

Add an InstSimplify for repetitive array expressions

I noticed in rust-lang/rust#135068 (comment) that GVN's implementation of this same transform was quite profitable on the deep-vector benchmark. But of course GVN doesn't run in unoptimized builds, so this is my attempt to write a version of this transform that benefits the deep-vector case and is fast enough to run in InstSimplify.

The benchmark suite indicates that this is effective.

Author: bors