[llvm] [ARM][Thumb1] Improve EstimateFunctionSizeInBytes accuracy (PR #203318)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

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

Full details are available at: https://lab.llvm.org/buildbot/#/builders/157/builds/48268

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

```
Step 6 (test-build-unified-tree-check-flang) failure: 1200 seconds without output running [b'ninja', b'check-flang'], attempting to kill
...
PASS: Flang :: Lower/OpenMP/requires-usm.f90 (4690 of 4702)
PASS: Flang :: Driver/mrecip.f90 (4691 of 4702)
PASS: Flang :: Transforms/vscale-attr.fir (4692 of 4702)
PASS: Flang :: Driver/fveclib.f90 (4693 of 4702)
PASS: Flang :: Driver/nostdlib.f90 (4694 of 4702)
PASS: Flang :: Driver/options-loongarch.f90 (4695 of 4702)
PASS: Flang :: Driver/mcmodel.f90 (4696 of 4702)
PASS: Flang :: Transforms/debug-dwarf-version.fir (4697 of 4702)
PASS: Flang :: Driver/fopenmp.f90 (4698 of 4702)
PASS: Flang :: Driver/use-module.f90 (4699 of 4702)
command timed out: 1200 seconds without output running [b'ninja', b'check-flang'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=3824.881291

```

</details>

https://github.com/llvm/llvm-project/pull/203318