riscv64: remove valueregs_2_reg extractor. (#5426)

This extractor had a side-effect of invoking `put_in_regs`, which is not
supposed to be invoked until the pattern-matching commits to evaluating
a rule right-hand side (i.e., cannot backtrack). In this case the
side-effect was mostly benign (in theory it could have caused additional
values to be computed needlessly), but in general we should be careful
to keep side-effects out of the left-hand side to enable further
optimizations and work on islec.

The implicit conversion from `Value` to `Reg` turns out to be enough to
make the rules in question work, so we can simply remove the use of the
extractor in this case.

cranelift/codegen/src/isa/riscv64/lower/isle.rs

@@ -315,10 +315,6 @@ impl generated_code::Context for IsleContext<'_, '_, MInst, Flags, IsaFlags, 6>
         self.isa_flags.has_zbkb()
     }
 
-    fn valueregs_2_reg(&mut self, val: Value) -> Reg {
-        self.put_in_regs(val).regs()[0]
-    }
-
     fn inst_output_get(&mut self, x: InstOutput, index: u8) -> ValueRegs {
         x[index as usize]
     }