Fix isa<> check which could never be true.

It was incorrectly checking a Function* being an IntrinsicInst* which
isn't possible.  It should always have been checking the CallInst* instead.

Added test case for x86 which ensures we only get one constant load.
It was 2 before this change.

rdar://problem/13267920

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175853 91177308-0d34-0410-b5e6-96231b3b80d8