amdgcn: HardFP divide

amdgcn: HardFP divide

Implement FP division using hardware instructions. This replaces both the
softfp library calls, and the --fast-math inaccurate divsion we had previously.

The GCN architecture does not have a single divide instruction, but it does
have a number of support instructions designed to make multiply-by-reciprocal
sufficiently accurate for non-fast-math usage.

gcc/ChangeLog:

	* config/gcn/gcn-valu.md (SV_SFDF): New iterator.
	(SV_FP): New iterator.
	(scalar_mode, SCALAR_MODE): Add identity mappings for scalar modes.
	(recip<mode>2): Unify the two patterns using SV_FP.
	(div_scale<mode><exec_vcc>): New insn.
	(div_fmas<mode><exec>): New insn.
	(div_fixup<mode><exec>): New insn.
	(div<mode>3): Unify the two expanders and rewrite using hardfp.
	* config/gcn/gcn.cc (gcn_md_reorg): Support "vccwait" attribute.
	* config/gcn/gcn.md (unspec): Add UNSPEC_DIV_SCALE, UNSPEC_DIV_FMAS,
	and UNSPEC_DIV_FIXUP.
	(vccwait): New attribute.

gcc/testsuite/ChangeLog:

	* gcc.target/gcn/fpdiv.c: Remove the -ffast-math requirement.