[mips] Optimize code generation for 64-bit variable shift instructions.

Summary:
The 64-bit version of the variable shift instructions uses the
shift_rotate_reg class which uses a GPR32Opnd to specify the variable
shift amount. With this patch we avoid the generation of a redundant
SLL instruction for the variable shift instructions in 64-bit targets.

Reviewers: dsanders

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D7413

llvm-svn: 235376