Rewrite EBB and value references after parsing.

We llow forward references to values and EBBs, so it is not possible to rewrite
these from the source domain to the in-memory domain during parsing.

Instead go through all the instructions after parsing everything and rewrite the
value and EBB references when everything has been created and mapped.

tests/parser/rewrite.cton

@@ -0,0 +1,24 @@
+; The .cton parser can't preserve the actual entity numbers in the input file
+; since entities are numbered as they are created. For entities declared in the
+; preamble, this is no problem, but for EBB and value references, mapping
+; source numbers to real numbers can be a problem.
+;
+; It is possible to refer to instructions and EBBs that have not yet been
+; defined in the lexical order, so the parser needs to rewrite these references
+; after the fact.
+
+; Check that defining numbers are rewritten.
+function defs() {
+ebb100(v20: i32):
+    v1000 = iconst.i32x8 5
+    vx200 = f64const 0x4.0p0
+    trap
+}
+
+; Using values.
+function use_value() {
+ebb100(v20: i32):
+    v1000 = iadd_imm v20, 5
+    vx200 = iadd v20, v1000
+    jump ebb100(v1000)
+}

tests/parser/rewrite.cton.ref

@@ -0,0 +1,13 @@
+function defs() {
+ebb0(vx0: i32):
+    v0 = iconst.i32x8 5
+    v1 = f64const 0x1.0000000000000p2
+    trap
+}
+
+function "use_value"() {
+ebb0(vx0: i32):
+    v0 = iadd_imm vx0, 5
+    v1 = iadd vx0, v0
+    jump ebb0(v0)
+}