Remove the use of -fast-isel with LLVM. (#2298)

This had already hit one bug in LLVM, and now I've hit another:
https://github.com/llvm/llvm-project/issues/58385

Since this isn't specific to a config, and it seems to be a clear sign
that this isn't the best tested path, let's just drop to the more fully
tested flags. Probably should have done this rather than the more
targeted workaround last time...

bazel/cc_toolchains/clang_cc_toolchain_config.bzl

@@ -268,19 +268,6 @@ def _impl(ctx):
                     "-O1",
                 ])],
             ),
-            # Use a conditional flag set for enabling the fast instruction
-            # selector to work around an LLVM bug:
-            # https://github.com/llvm/llvm-project/issues/56133
-            flag_set(
-                actions = codegen_compile_actions,
-                flag_groups = [flag_group(flags = [
-                    "-mllvm",
-                    "-fast-isel",
-                ])],
-                with_features = [
-                    with_feature_set(not_features = ["fuzzer"]),
-                ],
-            ),
         ],
     )
     default_optimization_flags = feature(