Revert "[AArch64][v8.3A] Avoid inserting implicit landing pads (PACI*SP)"

Linux kernel sets SCTRL_EL1.BT0 and BT1 to 1 unconditionally, which
makes PACIASP equivalent to BTI C + PACIA LR,SP.

Use the shorter instruction sequence by default.

I'm not aware of anyone who needs the opposite. They are welcome to
revert to the current behavior under a subtarget feature or an
environment check.

This reverts commit 571c8c52.

Differential Revision: https://reviews.llvm.org/D141978