Simplify branch arguments after splitting EBB arguments.

The EBB argument splitting may generate concat-split dependencies when
it repairs branch arguments in EBBs that have not yet been fully
legalized. Add a branch argument simplification step that can resolve
these dependency chains.

This means that all split and concatenation instructions will be dead
after legalization for types that have no legal instructions using them.

filetests/isa/riscv/legalize-abi.cton

@@ -36,7 +36,8 @@ ebb0:
     ; check: $(v1new=$V) = iconcat $v1l, $v1h
     ; check: $v1 = copy $v1new
     jump ebb1(v1)
-    ; check: jump $ebb1($v1)
+    ; The v1 copy gets resolved by split::simplify_branch_arguments().
+    ; check: jump $ebb1($v1new)
 
 ebb1(v10: i64):
     jump ebb1(v10)
@@ -50,9 +51,9 @@ ebb0:
     ; check: $ebb0:
     ; nextln: $v1, $(v2l=$V), $(v2h=$V) = call $fn1()
     ; check: $(v2new=$V) = iconcat $v2l, $v2h
-    ; check: $v2 -> $v2new
     jump ebb1(v1, v2)
-    ; check: jump $ebb1($v1, $v2)
+    ; The v2 -> v2new alias is resolved by split::simplify_branch_arguments().
+    ; check: jump $ebb1($v1, $v2new)
 
 ebb1(v9: i32, v10: i64):
     jump ebb1(v9, v10)
@@ -78,7 +79,8 @@ ebb0:
     ; check: $(v1new=$V) = ireduce.i8 $rv
     ; check: $v1 = copy $v1new
     jump ebb1(v1)
-    ; check: jump $ebb1($v1)
+    ; The v1 copy gets resolved by split::simplify_branch_arguments().
+    ; check: jump $ebb1($v1new)
 
 ebb1(v10: i8):
     jump ebb1(v10)

filetests/isa/riscv/split-args.cton

@@ -38,3 +38,18 @@ ebb3(v4: i64):
     return v5
     ; check: return $v5l, $v5h
 }
+
+function loop(i64, i64) -> i64 {
+ebb0(v1: i64, v2: i64):
+; check: $ebb0($(v1l=$V): i32, $(v1h=$V): i32, $(v2l=$V): i32, $(v2h=$V): i32):
+    jump ebb1(v1)
+    ; check: jump $ebb1($v1l, $v1h)
+
+ebb1(v3: i64):
+; check: $ebb1($(v3l=$V): i32, $(v3h=$V): i32):
+    v4 = band v3, v2
+    ; check: $(v4l=$V) = band $v3l, $v2l
+    ; check: $(v4h=$V) = band $v3h, $v2h
+    jump ebb1(v4)
+    ; check: jump $ebb1($v4l, $v4h)
+}