[mlir][spirv] Fix tablegen generator script's stripping of prefixes (#101378)

This script looks for existing definitions with the `SPIRV_` prefix, so
that it can preserve them when updating the file. When the commit
2d62833048 changed the prefix from `SPV_`,
the number of characters to strip from matched names was not updated,
which broke this feature. This commit fixes remaining cases that weren't
fixed by 339c87a8a0.

The relationship of this script to the files it is meant to maintain is
still bitrotten in other ways.

mlir/utils/spirv/gen_spirv_dialect.py

@@ -536,7 +536,10 @@ def gen_instr_coverage_report(path, instructions):
 
     content = content.split(AUTOGEN_OPCODE_SECTION_MARKER)
 
-    existing_opcodes = [k[11:] for k in re.findall("def SPIRV_OC_\w+", content[1])]
+    prefix = "def SPIRV_OC_"
+    existing_opcodes = [
+        k[len(prefix) :] for k in re.findall(prefix + "\w+", content[1])
+    ]
     existing_instructions = list(
         filter(lambda inst: (inst["opname"] in existing_opcodes), instructions)
     )
@@ -637,7 +640,12 @@ def update_td_enum_attrs(path, operand_kinds, filter_list):
     assert len(content) == 3
 
     # Extend filter list with existing enum definitions
-    existing_kinds = [k[8:-4] for k in re.findall("def SPIRV_\w+Attr", content[1])]
+    prefix = "def SPIRV_"
+    suffix = "Attr"
+    existing_kinds = [
+        k[len(prefix) : -len(suffix)]
+        for k in re.findall(prefix + "\w+" + suffix, content[1])
+    ]
     filter_list.extend(existing_kinds)
 
     capability_mapping = get_capability_mapping(operand_kinds)
@@ -959,12 +967,20 @@ def extract_td_op_info(op_def):
       - A dict containing potential manually specified sections
     """
     # Get opname
-    opname = [o[8:-2] for o in re.findall("def SPIRV_\w+Op", op_def)]
+    prefix = "def SPIRV_"
+    suffix = "Op"
+    opname = [
+        o[len(prefix) : -len(suffix)]
+        for o in re.findall(prefix + "\w+" + suffix, op_def)
+    ]
     assert len(opname) == 1, "more than one ops in the same section!"
     opname = opname[0]
 
     # Get instruction category
-    inst_category = [o[4:] for o in re.findall("SPIRV_\w+Op", op_def.split(":", 1)[1])]
+    prefix = "SPIRV_"
+    inst_category = [
+        o[len(prefix) :] for o in re.findall(prefix + "\w+Op", op_def.split(":", 1)[1])
+    ]
     assert len(inst_category) <= 1, "more than one ops in the same section!"
     inst_category = inst_category[0] if len(inst_category) == 1 else "Op"