Add heap_addr custom legalization.

The expansion of a heap_addr instruction depends on the type of heap and
its configuration, so this is handled by custom code.

Add a couple examples of heap access code to the language reference
manual.

cranelift/filetests/isa/intel/legalize-memory.cton

@@ -27,3 +27,26 @@ ebb1(v1: i64):
     return v2
     ; check: return $v2
 }
+
+; SpiderMonkey VM-style static 4+2 GB heap.
+; This eliminates bounds checks completely for offsets < 2GB.
+function %staticheap_sm64(i32, i64 vmctx) -> f32 spiderwasm {
+    gv0 = vmctx+64
+    heap0 = static gv0, min 0x1000, bound 0x1_0000_0000, guard 0x8000_0000
+
+ebb0(v0: i32, v999: i64):
+    ; check: $ebb0(
+    v1 = heap_addr.i64 heap0, v0, 1
+    ; Boundscheck should be eliminated.
+    ; Checks here are assuming that no pipehole opts fold the load offsets.
+    ; nextln: $(xoff=$V) = uextend.i64 $v0
+    ; nextln: $(haddr=$V) = iadd_imm $v999, 64
+    ; nextln: $(hbase=$V) = load.i64 $haddr
+    ; nextln: $v1 = iadd $hbase, $xoff
+    v2 = load.f32 v1+16
+    ; nextln: $v2 = load.f32 $v1+16
+    v3 = load.f32 v1+20
+    ; nextln: $v3 = load.f32 $v1+20
+    v4 = fadd v2, v3
+    return v4
+}