[mips] Move all ByVal handling into CCState and tablegen-erated code. NFC.

Summary:
CCState already contains a byval implementation that is very similar to the
Mips custom code. This patch merges the custom code into the existing
common code and tablegen-erated code.

Reviewers: vmedic

Reviewed By: vmedic

Subscribers: rnk, llvm-commits

Differential Revision: http://reviews.llvm.org/D5977