amdgcn: fix ICE on subreg of BI reg.

amdgcn: fix ICE on subreg of BI reg.

BImode usually only requires one bit, but instructions that write to VCC also
clobber the reset of the DImode register pair, so gcn_class_max_nregs reports
that two registers are needed for BImode.  Paradoxically, accessing VCC via
SImode is therefore uses fewer registers than accessing via BImode.

The LRA checking code takes this into account, but the DF liveness data also
looks at the subreg, so it says (subreg:SI (reg:BI VCC) 0) only makes the low
part live.  Both are "correct", but they disagree, which causes an ICE.

This doesn't happen when writing conditions to VCC; it happens when accessing
VCC_LO via a regular move to a regular SImode register.

If we transform the subregs so that BImode is always the outer mode then it
basically means the same thing, except that now both LRA and DF calculate nregs
the same, and ICE goes away.

As soon as LRA is done the subregs all evaporate anyway.

2020-02-27  Andrew Stubbs  <ams@codesourcery.com>

	gcc/
	* config/gcn/gcn.md (mov<mode>): Add transformations for BI subregs.