[arm] PR target/90405 fix regression for thumb1 with -mtpcs-leaf-frame

[arm] PR target/90405 fix regression for thumb1 with -mtpcs-leaf-frame

-mtpcs-leaf-frame causes an APCS-style backtrace frame to be created
on the stack.  This should probably be deprecated, but it did reveal
an issue with the patch I committed previously to improve the code
generation when pushing high registers, in that
thumb_find_work_register had a different idea as to which registers
were available as scratch registers.

The new code actually does a better job of finding a viable work
register and doesn't rely so much on assumptions about the ABI, so it
seems better to adapt thumb_find_work_register to the new approach.
This way we can eliminate some rather crufty code.

gcc:
	PR target/90405
	* config/arm/arm.c (callee_saved_reg_p): Move before
	thumb_find_work_register.
	(thumb1_prologue_unused_call_clobbered_lo_regs): Move before
	thumb_find_work_register.  Only call df_get_live_out once.
	(thumb1_epilogue_unused_call_clobbered_lo_regs): Likewise.
	(thumb_find_work_register): Use
	thumb1_prologue_unused_call_clobbered_lo_regs instead of ad hoc
	algorithms to locate a spare call clobbered reg.

gcc/testsuite:
	PR target/90405
	* gcc.target/arm/pr90405.c: New test.

From-SVN: r271036