[mypyc] feat: optimize away first index check in for loops if length > 1 (#19933)

It is not necessary to enter the gen_condition block on the first
iteration if the length is known to be positive, we can safely skip it. 
This won't be particularly impactful but its a low-hanging fruit.

Author: BobTheBuidler

mypyc/irbuild/for_helpers.py

@@ -183,7 +183,12 @@ def for_loop_helper_with_index(
 
     builder.push_loop_stack(step_block, exit_block)
 
-    builder.goto_and_activate(condition_block)
+    if isinstance(length, Integer) and length.value > 0:
+        builder.goto(body_block)
+        builder.activate_block(condition_block)
+    else:
+        builder.goto_and_activate(condition_block)
+
     for_gen.gen_condition()
 
     builder.activate_block(body_block)

mypyc/test-data/irbuild-lists.test

@@ -645,6 +645,7 @@ L0:
     r0 = 'abc'
     r1 = PyList_New(3)
     r2 = 0
+    goto L2
 L1:
     r3 = r2 < 3 :: signed
     if r3 goto L2 else goto L4 :: bool
@@ -690,6 +691,7 @@ L0:
     r0 = 'abc'
     r1 = PyList_New(3)
     r2 = 0
+    goto L2
 L1:
     r3 = r2 < 3 :: signed
     if r3 goto L2 else goto L4 :: bool
@@ -798,6 +800,7 @@ L0:
     r0 = b'abc'
     r1 = PyList_New(3)
     r2 = 0
+    goto L2
 L1:
     r3 = r2 < 3 :: signed
     if r3 goto L2 else goto L8 :: bool
@@ -952,6 +955,7 @@ L0:
     r18 = CPyList_Extend(r10, r17)
     r19 = PyList_New(13)
     r20 = 0
+    goto L2
 L1:
     r21 = var_object_size r10
     r22 = r20 < r21 :: signed

mypyc/test-data/irbuild-tuple.test

@@ -417,6 +417,7 @@ L0:
     r0 = 'abc'
     r1 = PyTuple_New(3)
     r2 = 0
+    goto L2
 L1:
     r3 = r2 < 3 :: signed
     if r3 goto L2 else goto L4 :: bool
@@ -462,6 +463,7 @@ L0:
     r0 = 'abc'
     r1 = PyTuple_New(3)
     r2 = 0
+    goto L2
 L1:
     r3 = r2 < 3 :: signed
     if r3 goto L2 else goto L4 :: bool
@@ -570,6 +572,7 @@ L0:
     r0 = b'abc'
     r1 = PyTuple_New(3)
     r2 = 0
+    goto L2
 L1:
     r3 = r2 < 3 :: signed
     if r3 goto L2 else goto L8 :: bool
@@ -879,6 +882,7 @@ L0:
     r18 = CPyList_Extend(r10, r17)
     r19 = PyTuple_New(13)
     r20 = 0
+    goto L2
 L1:
     r21 = var_object_size r10
     r22 = r20 < r21 :: signed