[AArch64][CodeGen] Avoid inverting hot branches during relaxation

Current behavior for relaxing out-of-range conditional branches
is to invert the conditional and insert a fallthrough unconditional
branch to the original destination. This approach biases the branch
predictor in the wrong direction, which can degrading performance.

Machine function splitting introduces many rarely-taken cross-section
conditional branches, which are improperly relaxed. Avoid inverting
these branches; instead, retarget them to trampolines at the end of the
function. Doing so increases the runtime cost of jumping to cold code
but eliminates the misprediction cost of jumping to hot code.

Differential Revision: https://reviews.llvm.org/D156837

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