[GVN] Avoid casting a vector of size less than 8 bits to i8

Summary:
A reprise of D25849.

This crash was found through fuzzing some time ago and was documented in PR28879.

No check for load size has been added due to the following tests:
  - Transforms/GVN/invariant.group.ll
  - Transforms/GVN/pr10820.ll

These tests expect load sizes that are not a multiple of eight.

Thanks to @davide for the original patch.

Reviewers: nlopes, davide, RKSimon, reames, efriedma

Reviewed By: efriedma

Subscribers: davide, llvm-commits, Prazek

Differential Revision: https://reviews.llvm.org/D48330

llvm-svn: 335294