sparc: Preserve ORIGINAL_REGNO in epilogue_renumber [PR105257]

The following testcase ICEs, because the pic register is
(reg:DI 24 %i0 [109]) and is used in the delay slot of a return.
We invoke epilogue_renumber and that changes it to
(reg:DI 8 %o0) which no longer satisfies sparc_pic_register_p
predicate, so we don't recognize the insn anymore.

The following patch fixes that by preserving ORIGINAL_REGNO if
specified, so we get (reg:DI 8 %o0 [109]) instead.

2022-04-19  Jakub Jelinek  <jakub@redhat.com>

	PR target/105257
	* config/sparc/sparc.cc (epilogue_renumber): If ORIGINAL_REGNO,
	use gen_raw_REG instead of gen_rtx_REG and copy over also
	ORIGINAL_REGNO.  Use return 0; instead of /* fallthrough */.

	* gcc.dg/pr105257.c: New test.