[llvm] [CodeGen] Add constexpr and static_assert to fold64To16 (PR #192864)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-debian-cpp20` running on `clang-debian-cpp20` while building `llvm` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/108/builds/25447

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

```
Step 6 (test-build-unified-tree-check-all) failure: test (failure)
...
PASS: LLVM :: CodeGen/AMDGPU/resource-usage-dead-function.ll (35892 of 89175)
PASS: LLVM :: CodeGen/AMDGPU/ret_jump.ll (35893 of 89175)
PASS: LLVM :: CodeGen/AMDGPU/returnaddress.ll (35894 of 89175)
PASS: LLVM :: CodeGen/AMDGPU/return-with-successors.mir (35895 of 89175)
PASS: LLVM :: CodeGen/AMDGPU/rewrite-out-arguments.ll (35896 of 89175)
PASS: LLVM :: CodeGen/AMDGPU/rewrite-undef-for-phi.ll (35897 of 89175)
PASS: LLVM :: CodeGen/AMDGPU/required-export-priority.ll (35898 of 89175)
PASS: LLVM :: CodeGen/AMDGPU/required-export-priority.mir (35899 of 89175)
PASS: LLVM :: CodeGen/AMDGPU/rewrite-vgpr-mfma-scale-to-agpr.mir (35900 of 89175)
UNRESOLVED: LLVM :: CodeGen/AMDGPU/rewrite-vgpr-mfma-to-agpr-spill-multi-store.s (35901 of 89175)
******************** TEST 'LLVM :: CodeGen/AMDGPU/rewrite-vgpr-mfma-to-agpr-spill-multi-store.s' FAILED ********************
Test has no 'RUN:' line
********************
PASS: LLVM :: CodeGen/AMDGPU/rewrite-vgpr-mfma-to-agpr-copy-from.mir (35902 of 89175)
PASS: LLVM :: CodeGen/AMDGPU/rewrite-partial-reg-uses.mir (35903 of 89175)
PASS: LLVM :: CodeGen/AMDGPU/rewrite-vgpr-mfma-to-agpr-phi.ll (35904 of 89175)
PASS: LLVM :: CodeGen/AMDGPU/repeated-divisor.ll (35905 of 89175)
PASS: LLVM :: CodeGen/AMDGPU/rewrite-partial-reg-uses-dbg.mir (35906 of 89175)
PASS: LLVM :: CodeGen/AMDGPU/ret.ll (35907 of 89175)
PASS: LLVM :: CodeGen/AMDGPU/rewrite-vgpr-mfma-to-agpr-subreg-insert-extract.mir (35908 of 89175)
PASS: LLVM :: CodeGen/AMDGPU/rewrite-vgpr-mfma-to-agpr.gfx90a.ll (35909 of 89175)
PASS: LLVM :: CodeGen/AMDGPU/rewrite-vgpr-mfma-to-agpr-subreg-src2-chain.mir (35910 of 89175)
PASS: LLVM :: CodeGen/AMDGPU/rotl.i64.ll (35911 of 89175)
PASS: LLVM :: CodeGen/AMDGPU/rewrite-vgpr-mfma-to-agpr-spill-multi-store.ll (35912 of 89175)
PASS: LLVM :: CodeGen/AMDGPU/rewrite-vgpr-mfma-to-agpr-spill-multi-store-mir.mir (35913 of 89175)
PASS: LLVM :: CodeGen/AMDGPU/rotate-add.ll (35914 of 89175)
PASS: LLVM :: CodeGen/AMDGPU/rotr.i64.ll (35915 of 89175)
PASS: LLVM :: CodeGen/AMDGPU/rv7x0_count3.ll (35916 of 89175)
PASS: LLVM :: CodeGen/AMDGPU/rewrite-vgpr-mfma-to-agpr.gfx950.ll (35917 of 89175)
PASS: LLVM :: CodeGen/AMDGPU/remat-vop.mir (35918 of 89175)
PASS: LLVM :: CodeGen/AMDGPU/rotl.ll (35919 of 89175)
PASS: LLVM :: CodeGen/AMDGPU/s-buffer-load-mmo-offsets.ll (35920 of 89175)
PASS: LLVM :: CodeGen/AMDGPU/s-barrier.ll (35921 of 89175)
PASS: LLVM :: CodeGen/AMDGPU/rotr.ll (35922 of 89175)
PASS: LLVM :: CodeGen/AMDGPU/s-barrier-lowering.ll (35923 of 89175)
PASS: LLVM :: CodeGen/AMDGPU/s-cluster-barrier.ll (35924 of 89175)
PASS: LLVM :: CodeGen/AMDGPU/s-getpc-b64-remat.ll (35925 of 89175)
PASS: LLVM :: CodeGen/AMDGPU/s_add_co_pseudo_lowering.mir (35926 of 89175)
PASS: LLVM :: CodeGen/AMDGPU/s-wakeup-barrier.ll (35927 of 89175)
PASS: LLVM :: CodeGen/AMDGPU/s_addk_i32.ll (35928 of 89175)
PASS: LLVM :: CodeGen/AMDGPU/rewrite-partial-reg-uses-gen.mir (35929 of 89175)
PASS: LLVM :: CodeGen/AMDGPU/s_cmp_0.ll (35930 of 89175)
PASS: LLVM :: CodeGen/AMDGPU/rsq.f32.ll (35931 of 89175)
PASS: LLVM :: CodeGen/AMDGPU/s_or_b32_transformation.ll (35932 of 89175)
PASS: LLVM :: CodeGen/AMDGPU/rsq.f32-safe.ll (35933 of 89175)
PASS: LLVM :: CodeGen/AMDGPU/s_movk_i32.ll (35934 of 89175)
PASS: LLVM :: CodeGen/AMDGPU/s_mulk_i32.ll (35935 of 89175)
PASS: LLVM :: CodeGen/AMDGPU/s_uaddo_usubo_pseudo.ll (35936 of 89175)
PASS: LLVM :: CodeGen/AMDGPU/same-lds-variable-multiple-use-in-one-phi-node.ll (35937 of 89175)

```

</details>

https://github.com/llvm/llvm-project/pull/192864