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.

compiler/asserts.go

@@ -186,6 +186,19 @@ func (b *builder) createNilCheck(inst ssa.Value, ptr llvm.Value, blockPrefix str
 	b.createRuntimeAssert(isnil, blockPrefix, "nilPanic")
 }
 
+// createNegativeShiftCheck creates an assertion that panics if the given shift value is negative.
+// This function assumes that the shift value is signed.
+func (b *builder) createNegativeShiftCheck(shift llvm.Value) {
+	if b.fn.IsNoBounds() {
+		// Function disabled bounds checking - skip shift check.
+		return
+	}
+
+	// isNegative = shift < 0
+	isNegative := b.CreateICmp(llvm.IntSLT, shift, llvm.ConstInt(shift.Type(), 0, false), "")
+	b.createRuntimeAssert(isNegative, "shift", "negativeShiftPanic")
+}
+
 // createRuntimeAssert is a common function to create a new branch on an assert
 // bool, calling an assert func if the assert value is true (1).
 func (b *builder) createRuntimeAssert(assert llvm.Value, blockPrefix, assertFunc string) {