Add nocapture to pointer parameters of masked stores/loads

The lack of this attribute (particularly on the load intrinsics)
prevented InstCombine from optimizing away allocas and memcpys
for arrays that could be read directly from rodata.

This now also includes a new test to check the masked load/store
intrinsics have the expected attributes (specifically nocapture).

Differential Revision: https://reviews.llvm.org/D135656