fmerge-all-constants does not work with constexpr non-literals

[slavanap]

Side-by-side comparison example

[llvmbot]

@llvm/issue-subscribers-clang-codegen

Author: Vyacheslav Napadovsky (slavanap)

[Side-by-side comparison example](https://godbolt.org/#z:OYLghAFBqd5QCxAYwPYBMCmBRdBLAF1QCcAaPECAMzwBtMA7AQwFtMQByARg9KtQYEAysib0QXACx8BBAKoBnTAAUAHpwAMvAFYTStJg1DIApACYAQuYukl9ZATwDKjdAGFUtAK4sGIMwDMpK4AMngMmAByPgBGmMQSGqQADqgKhE4MHt6%2B/kGp6Y4CYRHRLHEJXEl2mA6ZQgRMxATZPn6Btpj2RQwNTQQlUbHxibaNza25HQrjA%2BFD5SNVAJS2qF7EyOwc5gHhyN5YANQmAW5OM8SYrKfYJhoAgrv7h5gnZ5fhwAD6AG54mAA7rd7k9Ai8vMdTm4msQmABPEGPLzpIxHZhsBTJJhbI4zdCnKyPUFoBgzTCqZLEI6kmZHJheIg0rjvAAieII6BAIFhCOhyAQTVIADYQQB2CxgDgEKWkKWYWVShSK6UqyQqsxSkxi1mEkkCcmU6m0gj0xmoGlmNkcrk84hwxFnAVC0UBO4SqUy3jylXK72q/3q/2anY6vXEx7hU0sJjhCBR0jO4gAKmTy21RIeRxp61N0OhHOIXz%2BAMBEGQXAAdOgmI0IKsK5X0gAvTD15bvNwF1y0cNZnOMzsFz5GEtA8tmau1pj1xOTltt5Yd/NnI49vvZq4EDYMI4aPva1kcVa0TgAVl4fg4WlIqE4Xcs1jx602b12PFIBE0x9WAGsQAEk7CpIGhnmYIEABxnhBAQBBBkjCvonCSJe363pwvAKCASRftex6kHAsBIGgLDJHQ8TkJQJFkfQCTAAozDJAoCCoAQfB0AQ8RYRAMRoTE4RNPCnAfvxzDEPCADyMTaLUuEfiRbCCBJDC0EJeGkFgMReMAMK0LQWHcLwWAxkY4jqfgVx1L8mAGTeFK1Iy2wflGXRobQeAxHC4keFgaEEEWLDCbw1nEDEaSYKymAmcA7lGN%2BqxUAY9EAGqlhJySMEFMiCCIYjsFI2XyEoahoboXD6IYxjWNY%2BgeVhkCrKgyQ9AZAC0EnNkcrX4qcrKmI%2BlhmBo2atVQbDEMAmCtWItCtSahgEAovCoCFRZYPVs41HUzgQK4kx%2BOVoTzGUFR6AUGQCPtZ1pBdDCDCdSydN09SzFd5VbT0fTNPdwyVGM/Rvf933Hb9EirAoL5bGDSEcBepBXjed4cEcqgQcKrXAUcwDIMgRwQP5XgML%2BHYQLghAkCcgRcMsvC4VoS6kP%2BASSJWsFimKZ4aENaNnmKEEAJzSKeHAofDaFI5h2GfvFBEwIgKCoKR5FkBQ5aKzRIwHJVQ1cEwChKM07G0JxxDcbx6miYJWWW%2BJUkyQ4WUKYwBDKapaGadpun6VlxmVWZN4WbJeDWbZvD2cgjlZS5ws3u5nmCT52w3v5eCBYZpAhWFSiRdFsWgHhCVJQoqVAulmXp/wOWiOIBWV0VKjqOpuhmBVcX9VYli1TEG2Nc1mRtR1XU9QEfXVYNw2jeNk3TXpc0Go0ghLbeq14Ot8Dg10Qc7XtnhtHoR2lKD5XnT0gMn5kP2LH9H0vQDu%2B5O9m/bb0syX6d72vffB1A3Mh9X9DEMNhQ2pjDOGCNlqcBRmjDGkgaQGDRENSsVZhoQD1gbFqJMyZEGpO%2BGm0sC5/gAgESsXMuBijMLzUCnMIJmDFELZCqF1IS1sFLOmP4YZmEYYjDC%2BD6arBCukZwkggA%3D%3D)