+ MIPS: prevent coalescing written-to registers with $sp,$fp,$zero and $at.
+ Implemented subset of "spill replace" functionality, replacing moves from/to spilled registers with loads/stores to spill locations. This helps to reduce amount of instructions.

git-svn-id: http://svn.freepascal.org/svn/fpc/trunk@24900 3ad0048d-3df7-0310-abae-a5850022a9f2