[amdgcn] Fix handling of VCC_CONDITIONAL_REG

[amdgcn] Fix handling of VCC_CONDITIONAL_REG

Classify vcc_lo and vcc_hi into the VCC_CONDITIONAL_REG class,
and spill them into SGPRs if necessary.

2019-11-15  Kwok Cheung Yeung  <kcy@codesourcery.com>

	gcc/
	* config/gcn/gcn.c (gcn_regno_reg_class): Return VCC_CONDITIONAL_REG
	register class for VCC_LO and VCC_HI.
	(gcn_spill_class): Use SGPR_REGS to spill registers in
	VCC_CONDITIONAL_REG.

From-SVN: r278290