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.

src/runtime/panic.go

@@ -47,6 +47,11 @@ func chanMakePanic() {
 	runtimePanic("new channel is too big")
 }
 
+// Panic when a shift value is negative.
+func negativeShiftPanic() {
+	runtimePanic("negative shift")
+}
+
 func blockingPanic() {
 	runtimePanic("trying to do blocking operation in exported function")
 }