[llvm] [FastISel] Don't force SDAG fallback for libcalls (PR #171782)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `ppc64le-mlir-rhel-clang` running on `ppc64le-mlir-rhel-test` while building `llvm` at step 6 "test-build-check-mlir-build-only-check-mlir".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/129/builds/34828

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 6 (test-build-check-mlir-build-only-check-mlir) failure: 1200 seconds without output running [b'ninja', b'check-mlir'], attempting to kill
...
PASS: MLIR :: mlir-tblgen/op-interface.td (3743 of 3754)
PASS: MLIR :: mlir-tblgen/rewriter-errors.td (3744 of 3754)
PASS: MLIR-Unit :: Interfaces/./MLIRInterfacesTests/11/22 (3745 of 3754)
PASS: MLIR-Unit :: Interfaces/./MLIRInterfacesTests/13/22 (3746 of 3754)
PASS: MLIR-Unit :: Pass/./MLIRPassTests/11/14 (3747 of 3754)
PASS: MLIR :: mlir-tblgen/cpp-class-comments.td (3748 of 3754)
PASS: MLIR-Unit :: Interfaces/./MLIRInterfacesTests/12/22 (3749 of 3754)
PASS: MLIR :: mlir-runner/simple.mlir (3750 of 3754)
PASS: MLIR :: mlir-reduce/dce-test.mlir (3751 of 3754)
PASS: MLIR :: mlir-tblgen/llvm-intrinsics.td (3752 of 3754)
command timed out: 1200 seconds without output running [b'ninja', b'check-mlir'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=2127.085042

```

</details>

https://github.com/llvm/llvm-project/pull/171782