[MLIR] Fix generate-test-checks.py to not remove every blank lines (#166493)

The stripping of the notes was done on a line-by-line basis which was
fragile and led to remove empty lines everywhere in the file. Instead we
can strip it as a single block before splitting the input into multiple
lines.

Author: joker-eph

mlir/utils/generate-test-checks.py

@@ -230,14 +230,11 @@ def process_line(line_chunks, variable_namer, use_ssa_name=False, strict_name_re
 
 
 # Process the source file lines. The source file doesn't have to be .mlir.
-def process_source_lines(source_lines, note, args):
+def process_source_lines(source_lines, args):
     source_split_re = re.compile(args.source_delim_regex)
 
     source_segments = [[]]
     for line in source_lines:
-        # Remove previous note.
-        if line in note:
-            continue
         # Remove previous CHECK lines.
         if line.find(args.check_prefix) != -1:
             continue
@@ -359,9 +356,10 @@ def main():
 
     source_segments = None
     if args.source:
-        source_segments = process_source_lines(
-            [l.rstrip() for l in open(args.source, "r")], autogenerated_note, args
-        )
+        with open(args.source, "r") as f:
+            raw_source = f.read().replace(autogenerated_note, "")
+            raw_source_lines = [l.rstrip() for l in raw_source.splitlines()]
+        source_segments = process_source_lines(raw_source_lines, args)
 
     if args.inplace:
         assert args.output is None