Subzero: Improve handling of alloca instructions of constant size.

src/IceTargetLowering.h

Index: src/IceTargetLowering.h
diff --git a/src/IceTargetLowering.h b/src/IceTargetLowering.h
index 5ea7d31e25e76208dd9da3fc363c24fe7a03bf64..7f3eaa7b66d73905559f97f22b426367908009eb 100644
--- a/src/IceTargetLowering.h
+++ b/src/IceTargetLowering.h
@@ -359,7 +359,8 @@ protected:
   bool HasComputedFrame = false;
   bool CallsReturnsTwice = false;
   /// StackAdjustment keeps track of the current stack offset from its natural
-  /// location, as arguments are pushed for a function call.
+  /// location, e.g. as arguments are pushed for a function call or as
+  /// fixed-size alloca instructions are executed in the entry block.
   int32_t StackAdjustment = 0;
   SizeT NextLabelNumber = 0;
   SizeT NextJumpTableNumber = 0;