[flang] Enable FIR loop invariant code motion by default (#218703)
The FIR loop invariant code motion pass was added in #173438, where it
was disabled by default. This enables it for optimization levels above
`-O0`. The option `-disable-fir-licm` has been added in place of
`-enable-fir-licm`. The pass itself is unchanged.
Relates to #208086 and fixes #110613. Tests updated.
Assisted-by: Copilot