arm: Add GENERAL_AND_VPR_REGS regclass

arm: Add GENERAL_AND_VPR_REGS regclass

At some point during the development of this patch series, it appeared
that in some cases the register allocator wants “VPR or general”
rather than “VPR or general or FP” (which is the same thing as
ALL_REGS).  The series does not seem to require this anymore, but it
seems to be a good thing to do anyway, to give the register allocator
more freedom.

CLASS_MAX_NREGS and arm_hard_regno_nregs need adjustment to avoid a
regression in gcc.dg/stack-usage-1.c when compiled with -mthumb
-mfloat-abi=hard -march=armv8.1-m.main+mve.fp+fp.dp.

Most of the work of this patch series was carried out while I was
working at STMicroelectronics as a Linaro assignee.

2022-02-22  Christophe Lyon  <christophe.lyon@arm.com>

	gcc/
	* config/arm/arm.h (reg_class): Add GENERAL_AND_VPR_REGS.
	(REG_CLASS_NAMES): Likewise.
	(REG_CLASS_CONTENTS): Likewise.
	(CLASS_MAX_NREGS): Handle VPR.
	* config/arm/arm.cc (arm_hard_regno_nregs): Handle VPR.