[committed] Fix missing mode on a few unspec/unspec_volatile operands

[committed] Fix missing mode on a few unspec/unspec_volatile operands

This is fix for a minor problem Jivan and I found while testing the ext-dce work originally from Joern.

The ext-dce pass will transform zero/sign extensions into subreg accesses when
the upper bits are actually unused.  So it's more likely with the ext-dce work
to get a sequence like this prior to combine:

>
>> (insn 10 9 11 2 (set (reg:SI 144)
>>         (unspec_volatile [
>>                 (const_int 0 [0])
>>             ] UNSPECV_FRFLAGS)) "j.c":11:3 discrim 1 362 {riscv_frflags}
>>      (nil))
>> (insn 11 10 55 2 (set (reg:DI 140 [ _12 ])
>>         (subreg:DI (reg:SI 144) 0)) "j.c":11:3 discrim 1 206 {*movdi_64bit}
>>      (expr_list:REG_DEAD (reg:SI 144)
>>         (nil)))

When we try to combine insn 10->11 we'll ultimately call simplify_subreg with
something like

(subreg:DI (unspec_volatile [...]) 0)

Note the lack of a mode on the unspec_volatile.  That in turn will cause
simplify_subreg to trigger an assertion.

The modeless unspec is generated by the RISC-V backend and the more I've
pondered this issue over the last few days the more I'm convinced it's a
backend bug.  Basically if the LHS of the set has a mode, then the RHS of the
set should have a mode as well.

I've audited the various backends and only found a few problems which are fixed
by this patch.  I've tested the relevant ports in my tester.  c6x, sh, mips and
s390[x].

There are other patterns that are potentially problematical in various ports.
They have a REG destination and an UNSPEC source, but the REG has no mode in
the pattern.  Since it wasn't clear what mode to give the UNSPEC, I left those
alone.

gcc/

	* config/c6x/c6x.md (mvilc): Add mode to UNSPEC source.
	* config/mips/mips.md (rdhwr_synci_step_<mode>): Likewise.
	* config/riscv/riscv.md (riscv_frcsr, riscv_frflags): Likewise.
	* config/s390/s390.md (@split_stack_call<mode>): Likewise.
	(@split_stack_cond_call<mode>): Likewise.
	* config/sh/sh.md (sp_switch_1): Likewise.