[mips] abs.[ds], and neg.[ds] should be allowed regardless of -enable-no-nans-fp-math

Summary:
They behave in accordance with the Has2008 and ABS2008 configuration bits of the
processor which are used to select between the 1985 and 2008 versions of IEEE
754. In 1985 mode, these instructions are arithmetic (i.e. they raise invalid
operation exceptions when given NaN), in 2008 mode they are non-arithmetic
(i.e. they are copies).

nmadd.[ds], and nmsub.[ds] are still subject to -enable-no-nans-fp-math because
the ISA spec does not explicitly state that they obey Has2008 and ABS2008.

Reviewers: matheusalmeida

Reviewed By: matheusalmeida

Differential Revision: http://llvm-reviews.chandlerc.com/D3274