- MIPS: completely removed trgcpu.add_constraints method.
  While it can be correct from some point of view, it does not prevent aliasing a single-precision register into upper half of double-precision one. This is currently handled by making only even floating-point registers available to RA. At the same time, it somehow (possibly due to another bug in RA) generates a lot of unnecessary moves, as if physical double-precision registers conflict with each other. Anyway, removing it considerably improves the code without regressions in the test suite.

git-svn-id: http://svn.freepascal.org/svn/fpc/trunk@27999 3ad0048d-3df7-0310-abae-a5850022a9f2