Add a 'regalloc' filetest command.

Run functions through the register allocator, and then filecheck.

filetests/regalloc/basic.cton

@@ -0,0 +1,12 @@
+test regalloc
+
+; We can add more ISAs once they have defined encodings.
+isa riscv
+
+function add(i32, i32) {
+ebb0(v1: i32, v2: i32):
+    v3 = iadd v1, v2
+; check: [R#0c,%x0]
+; sameln: iadd
+    return_reg v3
+}