Add underflow/overflow tests.

tests/vm/toverflowopcaddimmint.nim

@@ -0,0 +1,11 @@
+discard """
+  errormsg: "over- or underflow"
+"""
+
+static:
+  proc p =
+    var
+      x = int64.high
+    discard x + 1
+    assert false
+  p()

tests/vm/toverflowopcaddint.nim

@@ -0,0 +1,12 @@
+discard """
+  errormsg: "over- or underflow"
+"""
+
+static:
+  proc p =
+    var
+      x = int64.high
+      y = 1
+    discard x + y
+    assert false
+  p()

tests/vm/toverflowopcmulint.nim

@@ -0,0 +1,11 @@
+discard """
+  errormsg: "over- or underflow"
+"""
+
+static:
+  proc p =
+    var
+      x = 1 shl 62
+    discard x * 2
+    assert false
+  p()

tests/vm/toverflowopcsubimmint.nim

@@ -0,0 +1,10 @@
+discard """
+  errormsg: "over- or underflow"
+"""
+
+static:
+  proc p =
+    var x = int64.low
+    discard x - 1
+    assert false
+  p()

tests/vm/toverflowopcsubint.nim

@@ -0,0 +1,12 @@
+discard """
+  errormsg: "over- or underflow"
+"""
+
+static:
+  proc p =
+    var
+      x = int64.low
+      y = 1
+    discard x - y
+    assert false
+  p()