Fix compFastTailCalls check. (#35596)

Fix compFastTailCalls check. (#35596)

compFastTailCalls is true if COMPlus_FastTailCalls is not 0.
It was introduced in #341 to help with testing:
setting COMPlus_FastTailCalls to 0 forces all tail-prefixed
calls to be dispatched via helpers.

This fixes a subtle bug with checking compFastTailCalls:
it needs to be checked after `fgInitargInfo` has been called in
`fgCanFastTailCall`. `fgInitArgInfo` adds the stub address for VSD calls
and `fgMorphTailCallViaHelpers` then removes it.

This change also factors out the logic for checking whether the call
has byref parameters that must be copied by the caller.