[NVPTX] Some nvvm.read.ptx.sreg intrinsics should have IntrInaccessibleMemOnly attribute.

These intrinsics may return different values every time they are called
and should not be CSE'd. IntrInaccessibleMemOnly appears to be the right
attribute to model this behavior.

Differential Revision: https://reviews.llvm.org/D57259

llvm-svn: 352256