[X86][SSE] Improve bool vector extload (PR26091)

We already have good codegen for (vXiY *ext(vXi1 bitcast(iX))) cases, this patch uses it for loads of vXi1 types as well - changing the load into a iX integer load, and bitcasting so that combineToExtendBoolVectorInReg can then use it.

Differential Revision: https://reviews.llvm.org/D62449

llvm-svn: 362081