mips.c (mips_expand_prologue): Use a single insn to allocate 32768 bytes of stack.

mips.c (mips_expand_prologue): Use a single insn to allocate 32768 bytes of stack.

	* config/mips/mips.c (mips_expand_prologue): Use a single insn to
	allocate 32768 bytes of stack.  Use addition rather than subtraction
	when a single insn is enough.
	* config/mips/mips.md: Remove insns and splitters for subtracting
	constants.
	(subsi3): Only accept register operands.
	(subsi3_internal): Likewise.  Use for TARGET_MIPS16 as well.
	(subdi3_internal_3, subsi3_internal_2): Likewise.
	(casesi): Use expand_binop to subtract the lower bound.

From-SVN: r69357