[AArch64] Move machine bundle unpacking to PreEmit2 phase.

This patch adjusts the placement of the bundle unpacking to just before
code emission. In particular, this means bundle unpacking happens AFTER
the machine outliner. With the previous position, the machine outliner
may outline parts of a bundle, which breaks them up.

This is an issue for BLR_RVMARKER handling, as illustrated by the
rvmarker-pseudo-expansion-and-outlining.mir test case. The machine
outliner should not break up the bundles created during pseudo
expansion.

This should fix PR49082.

Reviewed By: SjoerdMeijer

Differential Revision: https://reviews.llvm.org/D96294