compiler: properly implement div and rem operations

The division and remainder operations were lowered directly to LLVM IR.
This is wrong however because the Go specification defines exactly what
happens on a divide by zero or signed integer overflow and LLVM IR
itself treats those cases as undefined behavior. Therefore, this commit
implements divide by zero and signed integer overflow according to the
Go specification.

This does have an impact on the generated code, but it is surprisingly
small. I've used the drivers repo to test the code before and after, and
to my surprise most driver smoke tests are not changed at all. Those
that are, have only a small increase in code size. At the same time,
this change makes TinyGo more compliant to the Go specification.

testdata/binop.go

@@ -75,6 +75,10 @@ func main() {
 	println("constant number")
 	x := uint32(5)
 	println(uint32(x) / (20e0 / 1))
+
+	// check for signed integer overflow
+	println("-2147483648 / -1:", sdiv32(-2147483648, -1))
+	println("-2147483648 % -1:", srem32(-2147483648, -1))
 }
 
 var x = true
@@ -114,6 +118,14 @@ func ashr(x int, y uint) int {
 	return x >> y
 }
 
+func sdiv32(x, y int32) int32 {
+	return x / y
+}
+
+func srem32(x, y int32) int32 {
+	return x % y
+}
+
 var shlSimple = shl(2, 1)
 var shlOverflow = shl(2, 1000)
 var shrSimple = shr(2, 1)

testdata/binop.txt

@@ -64,3 +64,5 @@ true
 true
 constant number
 0
+-2147483648 / -1: -2147483648
+-2147483648 % -1: 0