amdgcn: Add clrsbsi2/clrsbdi2 implementation

amdgcn: Add clrsbsi2/clrsbdi2 implementation

This patch adds an open-coded implementation of the clrsb<mode>2
(count leading redundant sign bit) standard names using the GCN flbit_i*
instructions for SImode and DImode.  Those don't count exactly as we need,
so we need a couple of other instructions to fix up the result afterwards.

These patterns are lost from libgcc if we build it for DImode/TImode
rather than SImode/DImode, a change we make in a later patch in this
series.

2021-06-18  Julian Brown  <julian@codesourcery.com>

gcc/
	* config/gcn/gcn.md (UNSPEC_FLBIT_INT): New unspec constant.
	(s_mnemonic): Add clrsb.
	(gcn_flbit<mode>_int): Add insn pattern for SImode/DImode.
	(clrsb<mode>2): Add expander for SImode/DImode.