[AddDiscriminators] Assign discriminators to MemIntrinsic calls.

Before this patch, pass AddDiscriminators always avoided to assign
discriminators to intrinsic calls. This was done mainly for two reasons:
 1) We wanted to minimize the number of based discriminators used.
 2) We wanted to avoid non-deterministic discriminator assignment for
    different debug levels.

Unfortunately, that approach was problematic for MemIntrinsic calls.
MemIntrinsic calls can be split by SROA into loads and stores, and each new
load/store instruction would obtain the debug location from the original
intrinsic call.
If we don't assign a discriminator to MemIntrinsic calls, then we cannot
correctly set the discriminator for the newly created loads and stores.
This may have a negative impact on the basic block weight computation
performed by the SampleLoader.

This patch fixes the issue by letting MemIntrinsic calls have a discriminator.

Differential Revision: https://reviews.llvm.org/D31900

llvm-svn: 299972