compiler: implement spec-compliant shifts

Previously, the compiler used LLVM's shift instructions directly, which have UB whenever the shifts are large or negative.
This commit adds runtime checks for negative shifts, and handles oversized shifts.

testdata/binop.go

@@ -61,6 +61,15 @@ func main() {
 	println(c128 != 3+2i)
 	println(c128 != 4+2i)
 	println(c128 != 3+3i)
+
+	println("shifts")
+	println(shlSimple == 4)
+	println(shlOverflow == 0)
+	println(shrSimple == 1)
+	println(shrOverflow == 0)
+	println(ashrNeg == -1)
+	println(ashrOverflow == 0)
+	println(ashrNegOverflow == -1)
 }
 
 var x = true
@@ -87,3 +96,23 @@ type Struct2 struct {
 	_ float64
 	i int
 }
+
+func shl(x uint, y uint) uint {
+	return x << y
+}
+
+func shr(x uint, y uint) uint {
+	return x >> y
+}
+
+func ashr(x int, y uint) int {
+	return x >> y
+}
+
+var shlSimple = shl(2, 1)
+var shlOverflow = shl(2, 1000)
+var shrSimple = shr(2, 1)
+var shrOverflow = shr(2, 1000000)
+var ashrNeg = ashr(-1, 1)
+var ashrOverflow = ashr(1, 1000000)
+var ashrNegOverflow = ashr(-1, 1000000)

testdata/binop.txt

@@ -54,3 +54,11 @@ false
 true
 true
 true
+shifts
+true
+true
+true
+true
+true
+true
+true