[mips] correct operand range for DINSM instruction

This patch corrects the definition of the DINSM instruction.
Specification for DINSM instruction for Mips64 says that size operand should
be 2 <= size <= 64, but it is defined as uimm5_inssize_plus1 which gives
range of 1 .. 32.

Patch by Aleksandar Beserminji.

Differential Revision: https://reviews.llvm.org/D37683

llvm-svn: 313149