[mips][msa] Remove copy_u.d and move copy_u.w to MSA64.

Summary:
The forwards compatibility strategy employed by MIPS is to consider registers
to be infinitely sign-extended. Then on ISA's with a wider register, the result
of existing instructions are sign-extended to register width and zero-extended
counterparts are added. copy_u.w on MSA32 and copy_u.w on MSA64 violate this
strategy and we have therefore corrected the MSA specs to fix this.

We still keep track of sign/zero-extension during legalization but we now
match copy_s.[wd] where required.

No change required to clang since __builtin_msa_copy_u_[wd] will map to
copy_s.[wd] where appropriate for the target.

Reviewers: vkalintiris

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D13472

llvm-svn: 250887