[instcombine] (extelt (inselt Vec, Value, Index), Index) -> Value

When Index is variable but still trivially known to be equal we can use Value
from before the insertion, possibly eliminating the vector.

Reverts a functional change from:
Author: Philip Reames <listmail@philipreames.com>
Date:   Wed Dec 8 12:21:10 2021 -0800

    [instcombine] A couple style tweaks to visitExtractElementInst [nfc]

Thanks to Michele Scandale for identifying the bug

Differential Revision: https://reviews.llvm.org/D135625