[mips] Check the register class before replacing materializations of zero with $zero...

[mips] Check the register class before replacing materializations of zero with $zero in microMIPS.

Summary:
The microMIPS register class GPRMM16 does not contain the $zero register.
However, MipsSEDAGToDAGISel::replaceUsesWithZeroReg() would replace uses
of the $dst register:

  [d]addiu, $dst, $zero, 0

with the $zero register, without checking for membership in the register
class of the target machine operand.

Reviewers: dsanders

Subscribers: llvm-commits, dsanders

Differential Revision: http://reviews.llvm.org/D13984

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@251622 91177308-0d34-0410-b5e6-96231b3b80d8