compiler: simplify code around getZeroValue

The LLVM library we use does not (yet) provide a llvm.Zero (like it
provides a llvm.Undef) so we have implemented our own. However, in
theory it might return an error in some cases.

No real-world errors have been seen in a while and errors would likely
indicate a serious compiler bug anyway (not an external error), so make
it panic instead of returning an error.

compiler/defer.go

@@ -136,10 +136,7 @@ func (c *Compiler) emitDefer(frame *Frame, instr *ssa.Defer) error {
 
 	// Make a struct out of the collected values to put in the defer frame.
 	deferFrameType := c.ctx.StructType(valueTypes, false)
-	deferFrame, err := c.getZeroValue(deferFrameType)
-	if err != nil {
-		return err
-	}
+	deferFrame := c.getZeroValue(deferFrameType)
 	for i, value := range values {
 		deferFrame = c.builder.CreateInsertValue(deferFrame, value, i, "")
 	}