Syntax tweak: Omit comma after an initial enum immediate.

This affects the comparison instructions which now read "icmp ult a, b".
This mimics LLVM's style and makes it simpler to add instruction flags
in the future, such as "load v1" -> "load aligned v1".

These enumerated operands and flags feel like opcode modifiers rather
than value operands, so displaying them differently makes sense.

Value and numeric operands are still comma separated.

filetests/isa/riscv/expand-i32.cton

@@ -15,7 +15,7 @@ ebb0(v1: i32, v2: i32):
     return v3, v4
 }
 ; check: $v3 = iadd $v1, $v2
-; check: $(cout=$V) = icmp ult, $v3, $v1
+; check: $(cout=$V) = icmp ult $v3, $v1
 ; It's possible the legalizer will rewrite these value aliases in the future.
 ; check: $v4 -> $cout
 ; check: return $v3, $v4