RISC-V: Add RTL pass to combine cm.popret with zero return value

This patch implements a new RTL pass that combines "li a0, 0" and
"cm.popret" into a single "cm.popretz" instruction for the Zcmp
extension.

This optimization cannot be done during prologue/epilogue expansion
because it would cause shrink-wrapping to generate incorrect code as
documented in PR113715. The dedicated RTL pass runs after shrink-wrap
but before branch shortening, safely performing this combination.

Changes since v2:
- Apply Jeff's comment
  - Use CONST0_RTX rather than const0_rtx, this make this pass able to
    handle (const_double:SF 0.0) as well.
- Adding test case for float/double zero return value.
Changes since v1:
- Tweak the testcase.

gcc/ChangeLog:

	* config/riscv/riscv-opt-popretz.cc: New file.
	* config/riscv/riscv-passes.def: Insert pass_combine_popretz before
	pass_shorten_branches.
	* config/riscv/riscv-protos.h (make_pass_combine_popretz): New
	declaration.
	* config/riscv/t-riscv: Add riscv-opt-popretz.o build rule.
	* config.gcc (riscv*): Add riscv-opt-popretz.o to extra_objs.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/pr113715.c: New test.
	* gcc.target/riscv/rv32e_zcmp.c: Update expected output for
	test_popretz.
	* gcc.target/riscv/rv32i_zcmp.c: Likewise.

0 files changed, 0 insertions, 0 deletions