Legalize entry block arguments to match ABI types.

Insert conversion code that reconstructs the original function argument
types from the legalized ABI signature.

Add abi::legalize_abi_value(). This function is used when adapting code
to a legalized function signature.

cranelift/filetests/isa/riscv/abi.cton

@@ -2,6 +2,9 @@
 test legalizer
 isa riscv
 
+; regex: V=v\d+
+; regex: VX=vx\d+
+
 function f(i32) {
     sig0 = signature(i32) -> i32
 
@@ -21,3 +24,11 @@ function f(i32) {
 ebb0(v0: i32):
     return_reg v0
 }
+
+function int_split_args(i64) -> i64 {
+ebb0(v0: i64):
+; check: $ebb0($(v0l=$VX): i32, $(v0h=$VX): i32):
+; check: iconcat_lohi $v0l, $v0h
+    v1 = iadd_imm v0, 1
+    return v0
+}