[AVX512] Remove leftover code for when i1 was a legal type from the fast isel load/store code.

Summary:
I don't think we need this code anymore. It only existed because i1 used to be legal.

There's probably more unneeded code in fast isel still.

Reviewers: guyblank, zvi

Reviewed By: guyblank

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D36652

llvm-svn: 310843