compiler: lower interfaces in a separate pass

This commit changes many things:

  * Most interface-related operations are moved into an optimization
    pass for more modularity. IR construction creates pseudo-calls which
    are lowered in this pass.
  * Type codes are assigned in this interface lowering pass, after DCE.
  * Type codes are sorted by usage: types more often used in type
    asserts are assigned lower numbers to ease jump table construction
    during machine code generation.
  * Interface assertions are optimized: they are replaced by constant
    false, comparison against a constant, or a typeswitch with only
    concrete types in the general case.
  * Interface calls are replaced with unreachable, direct calls, or a
    concrete type switch with direct calls depending on the number of
    implementing types. This hopefully makes some interface patterns
    zero-cost.

These changes lead to a ~0.5K reduction in code size on Cortex-M for
testdata/interface.go. It appears that a major cause for this is the
replacement of function pointers with direct calls, which are far more
susceptible to optimization. Also, not having a fixed global array of
function pointers greatly helps dead code elimination.

This change also makes future optimizations easier, like optimizations
on interface value comparisons.

interp/frame.go

@@ -300,6 +300,8 @@ func (fr *frame) evalBasicBlock(bb, incoming llvm.BasicBlock, indent string) (re
 				ret = llvm.ConstInsertValue(ret, retLen, []uint32{1}) // len
 				ret = llvm.ConstInsertValue(ret, retLen, []uint32{2}) // cap
 				fr.locals[inst] = &LocalValue{fr.Eval, ret}
+			case callee.Name() == "runtime.makeInterface":
+				fr.locals[inst] = &LocalValue{fr.Eval, llvm.ConstPtrToInt(inst.Operand(0), fr.TargetData.IntPtrType())}
 			case strings.HasPrefix(callee.Name(), "runtime.print") || callee.Name() == "runtime._panic":
 				// all print instructions, which necessarily have side
 				// effects but no results

interp/scan.go

@@ -73,7 +73,12 @@ func (e *Eval) hasSideEffects(fn llvm.Value) *sideEffectResult {
 					result.updateSeverity(sideEffectAll)
 					continue
 				}
+				name := child.Name()
 				if child.IsDeclaration() {
+					if name == "runtime.makeInterface" {
+						// Can be interpreted so does not have side effects.
+						continue
+					}
 					// External function call. Assume only limited side effects
 					// (no affected globals, etc.).
 					if result.hasLocalSideEffects(dirtyLocals, inst) {