TableGen: Add !size operation

Summary:
Returns the size of a list. I have found this to be rather useful in some
development for the AMDGPU backend where we could simplify our .td files
by concatenating list<LLVMType> for complex intrinsics. Doing so requires
us to compute the position argument for LLVMMatchType.

Basically, the usage is in a pattern that looks somewhat like this:

    list<LLVMType> argtypes =
        !listconcat(base,
                    [llvm_any_ty, LLVMMatchType<!size(base)>]);

Change-Id: I360a0b000fd488d18bea412228230fd93722bd2c

Reviewers: arsenm, craig.topper, tra, MartinO

Subscribers: wdng, llvm-commits, tpr

Differential Revision: https://reviews.llvm.org/D43553

llvm-svn: 325883