expr: don't clear SUBREG_PROMOTED_VAR_P flag for a promoted subreg [target/111466]

expr: don't clear SUBREG_PROMOTED_VAR_P flag for a promoted subreg [target/111466]

RISC-V suffers from extraneous sign extensions, despite/given the ABI
guarantee that 32-bit quantities are sign-extended into 64-bit registers,
meaning incoming SI function args need not be explicitly sign extended
(so do SI return values as most ALU insns implicitly sign-extend too.)

Existing REE doesn't seem to handle this well and there are various ideas
floating around to smarten REE about it.

RISC-V also seems to correctly implement middle-end hook PROMOTE_MODE
etc.

Another approach would be to prevent EXPAND from generating the
sign_extend in the first place which this patch tries to do.

The hunk being removed was introduced way back in 1994 as
   5069803972 ("expand_expr, case CONVERT_EXPR .. clear the promotion flag")

This survived full testsuite run for RISC-V rv64gc with surprisingly no
fallouts: test results before/after are exactly same.

|                               | # of unexpected case / # of unique unexpected case
|                               |          gcc |          g++ |     gfortran |
| rv64imafdc_zba_zbb_zbs_zicond/|  264 /    87 |    5 /     2 |   72 /    12 |
|    lp64d/medlow

Granted for something so old to have survived, there must be a valid
reason. Unfortunately the original change didn't have additional
commentary or a test case. That is not to say it can't/won't possibly
break things on other arches/ABIs, hence the RFC for someone to scream
that this is just bonkers, don't do this 🙂

I've explicitly CC'ed Jakub and Roger who have last touched subreg
promoted notes in expr.cc for insight and/or screaming 😉

Thanks to Robin for narrowing this down in an amazing debugging session
@ GNU Cauldron.

```
foo2:
	sext.w	a6,a1             <-- this goes away
	beq	a1,zero,.L4
	li	a5,0
	li	a0,0
.L3:
	addw	a4,a2,a5
	addw	a5,a3,a5
	addw	a0,a4,a0
	bltu	a5,a6,.L3
	ret
.L4:
	li	a0,0
	ret
```

Signed-off-by: Vineet Gupta <vineetg@rivosinc.com>
Co-developed-by: Robin Dapp <rdapp.gcc@gmail.com>
	PR target/111466
gcc/
	* expr.cc (expand_expr_real_2): Do not clear SUBREG_PROMOTED_VAR_P.

gcc/testsuite
	* gcc.target/riscv/pr111466.c: New test.