[ARM] Avoid redundant checks for isThumb1Only() after supportsTailCall()

supportsTailCall() has two callers. Both of them double-check isThumb1Only(),
and refuse to proceed with tail-calling in that case.
Therefore, it makes sense to move this check to
ARMSubtarget::initSubtargetFeatures, where SupportsTailCall is initialized;
and to eliminate the extra checks at the call sites.

Following a review comment, added an "assert(supportsTailCall())"
in IsEligibleForTailCall.

NFC.

llvm-svn: 248703