cse: Take single register constraints into account

cse: Take single register constraints into account

This fixes

t.c:17:3: error: 'asm' operand has impossible constraints or there are not enough registers
   17 |   __asm__ ("" : "=f" (a), "={fr2}" (e) : "{fr1}" (d));
      |   ^~~~~~~

on powerpc64le-unknown-linux-gnu for the attached test.  Prior cse1 we have

(insn 2 4 3 2 (set (reg/v:TF 120 [ c ])
        (reg:TF 33 1 [ c ])) "t.c":14:26 614 {*movtf_64bit_dm}
     (nil))
...
(insn 10 9 6 2 (parallel [
            (set (reg:DF 121)
                (asm_operands:DF ("") ("=f") 0 [
                        (reg:TF 124)
                    ]
                     [
                        (asm_input:TF ("{fr1}") t.c:17)
                    ]
                     [] t.c:17))
            (set (reg:DF 123 [ e ])
                (asm_operands:DF ("") ("={fr2}") 1 [
                        (reg:TF 124)
                    ]
                     [
                        (asm_input:TF ("{fr1}") t.c:17)
                    ]
                     [] t.c:17))
            (clobber (reg:SI 98 ca))
        ]) "t.c":17:3 -1
     (nil))
...
(insn 12 11 13 2 (set (reg:TF 33 1)
            (reg/v:TF 120 [ c ])) "t.c":18:12 614 {*movtf_64bit_dm}
     (nil))

During cse1, in insn 12 pseudo 120 is substituted with hard register 33
rendering the resulting insn trivial which is why the insn gets removed
afterwards.  Since hard register 33 has a use after insn 12, the
register is live before and after insn 10.  This leaves us with the
non-trivial problem, during LRA, to also assign hard register 33 to
pseudo 124 which is coming from the constraint of insn 10.  Since hard
registers are not tracked, except for liveness, this cannot be solved by
reloads which is why we end up with an error.  Therefore, treat single
register constraints as clobbers of the respective hard registers.

For the sake of symmetry this should also be done for constraints
associated a single register class.  However, since we are in stage 4,
there is no open PR, and I haven't done any extensive testing for single
register classes, I'm skipping this for the moment.  Once we are back in
stage 1, something along the lines could be added:

        else
          {
            enum reg_class cl
              = reg_class_for_constraint (lookup_constraint (p));
            if (cl == NO_REGS)
              continue;
            machine_mode mode = recog_data.operand_mode[nop];
            int regno = ira_class_singleton[cl][mode];
            if (regno >= 0)
              invalidate_reg (gen_rtx_REG (mode, regno));
          }

gcc/ChangeLog:

	* cse.cc (invalidate_from_sets_and_clobbers): Consider any hard
	register referred to by any single register constraint
	potentially being clobbered.

gcc/testsuite/ChangeLog:

	* gcc.target/powerpc/asm-hard-reg-2.c: New test.