Advance the insertion cursor after replacinf an instruction.

When expanding iadd_cout, the original instruction is replaced with an
iadd, and an icmp is inserted after the iadd.

Make sure we advance the insertion position after replacing iadd_cout so
the icmp gets inserted *after* iadd.

cranelift/filetests/isa/riscv/expand-i32.cton

@@ -0,0 +1,18 @@
+; Test the legalization of i32 instructions that don't have RISC-V versions.
+test legalizer
+
+set is_64bit=0
+isa riscv supports_m=1
+
+set is_64bit=1
+isa riscv supports_m=1
+
+; regex: V=vx?\d+
+
+function carry_out(i32, i32) -> i32, b1 {
+ebb0(v1: i32, v2: i32):
+    v3, v4 = iadd_cout v1, v2
+    return v3, v4
+}
+; check: $v3 = iadd $v1, $v2
+; check: $(cout=$V) = icmp ult, $v3, $v1