[NFC][IndVarSimplify] remove duplicate code in widenWithVariantLoadUseCodegen.

Summary: Duplicate code in widenWithVariantLoadUseCodegen is removed and also use assert to check unknown extension type as it should be filtered out by the pre condition check before calling this function.

Reviewers: az, sanjoy, sebpop, efriedma, javed.absar, sanjoy.google

Reviewed By: efriedma

Subscribers: hiraditya, llvm-commits, amehsan

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72652