compiler: implement most math/bits functions

These functions can be implemented more efficiently using LLVM
intrinsics. That makes them the Go equivalent of functions like
__builtin_clz which are also implemented using these LLVM intrinsics.

I believe the Go compiler does something very similar: IIRC it converts
calls to these functions into optimal instructions for the given
architecture.

I tested these by running `tinygo test math/bits` after uncommenting the
tests that would always fail (the *PanicZero and *PanicOverflow tests).

compiler/llvm.go

@@ -464,6 +464,19 @@ func (b *builder) readStackPointer() llvm.Value {
 	return b.CreateCall(stacksave.GlobalValueType(), stacksave, nil, "")
 }
 
+// createZExtOrTrunc lets the input value fit in the output type bits, by zero
+// extending or truncating the integer.
+func (b *builder) createZExtOrTrunc(value llvm.Value, t llvm.Type) llvm.Value {
+	valueBits := value.Type().IntTypeWidth()
+	resultBits := t.IntTypeWidth()
+	if valueBits > resultBits {
+		value = b.CreateTrunc(value, t, "")
+	} else if valueBits < resultBits {
+		value = b.CreateZExt(value, t, "")
+	}
+	return value
+}
+
 // Reverse a slice of bytes. From the wiki:
 // https://github.com/golang/go/wiki/SliceTricks#reversing
 func reverseBytes(buf []byte) {