[mips] Specify the correct value type when combining a CMovFP node.

This commit fixes a bug introduced in r230956 where we were creating
CMovFP_{T,F} nodes with multiple return value types (one for each operand).
With this change the return value type of the new node is the same as the
value type of the True/False operands of the original node.

llvm-svn: 231237