Fix another SROA crasher, PR14601.
This was a silly oversight, we weren't pruning allocas which were used
by variable-length memory intrinsics from the set that could be widened
and promoted as integers. Fix that.
This should fix the following assertion failure:
Assertion failed: (CanSROA), function visitUsers, file
/usr/src/lib/clang/libllvmscalaropts/../../../contrib/llvm/lib/Transforms/Scalar/SROA.cpp,
line 2395.
Reported by: gerald