Optimized loading (zextload) of i1 value from memory.

This patch is a partial revert of https://llvm.org/svn/llvm-project/llvm/trunk@237793.
Extra "and" causes performance degradation.

We assume that i1 is stored in zero-extended form. And store operation is responsible for zeroing upper bits.

Differential Revision: http://reviews.llvm.org/D17541

llvm-svn: 261828