[MIPS] Fix register renaming in the interrupt handlers.

[MIPS] Fix register renaming in the interrupt handlers.

gcc/
	* config/mips/mips-protos.h (mips_hard_regno_rename_ok): New prototype.
	* config/mips/mips.c (mips_hard_regno_rename_ok): New function.
	(mips_hard_regno_scratch_ok): Likewise.
	(TARGET_HARD_REGNO_SCRATCH_OK): Define macro.
	* config/mips/mips.h (HARD_REGNO_RENAME_OK): New.

gcc/testsuite/
	* gcc.target/mips/interrupt_handler-bug-1.c: New test.

From-SVN: r226968