[AArch64][v8.5A] Restrict indirect tail calls to use x16/17 only when using BTI

When branch target identification is enabled, all indirectly-callable
functions start with a BTI C instruction. this instruction can only be
the target of certain indirect branches (direct branches and
fall-through are not affected):
- A BLR instruction, in either a protected or unprotected page.
- A BR instruction in a protected page, using x16 or x17.
- A BR instruction in an unprotected page, using any register.

Without BTI, we can use any non call-preserved register to hold the
address for an indirect tail call. However, when BTI is enabled, then
the code being compiled might be loaded into a BTI-protected page, where
only x16 and x17 can be used for indirect tail calls.

Legacy code withiout this restriction can still indirectly tail-call
BTI-protected functions, because they will be loaded into an unprotected
page, so any register is allowed.

Differential revision: https://reviews.llvm.org/D52868

llvm-svn: 343968

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