[ELF] Rename canRelax to toExecRelax. NFC

In the absence of TLS relaxation (rewrite of code sequences),
there is still an applicable optimization:

[gd]: General Dynamic: resolve DTPMOD to 1 and/or resolve DTPOFF statically

All the other relaxations are only performed when transiting to
executable (`!config->shared`).
Since [gd] is handled differently, we can fold `!config->shared` into canRelax
and simplify its use sites. Rename the variable to reflect to new semantics.

Reviewed By: grimar, psmith

Differential Revision: https://reviews.llvm.org/D83243