CodeGenPrepare: Add support for llvm.threadlocal.address address-mode sinking (#87844)

Depending on the TLSMode many thread-local accesses on x86 can be
expressed by adding a %fs: segment register to an addressing mode. Even
if there are mutliple users of a `llvm.threadlocal.address` intrinsic it
is generally not worth sharing the value in a register but instead fold
the %fs access into multiple addressing modes.

Hence this changes CodeGenPrepare to duplicate the
`llvm.threadlocal.address` intrinsic as necessary.

Introduces a new `TargetLowering::addressingModeSupportsTLS` callback
that allows targets to indicate whether TLS accesses can be part of an
addressing mode.

This is fixing a performance problem, as this folding of TLS-accesses
into multiple addressing modes happened naturally before the
introduction of the `llvm.threadlocal.address` intrinsic, but regressed
due to `SelectionDAG` keeping things in registers when accessed across
basic blocks, so CodeGenPrepare needs to duplicate to mitigate this. We
see a ~0.5% recovery in a codebase with heavy TLS usage (HHVM).

This fixes most of #87437