[CFIFixup] Allow function prologues to span more than one basic block (#68984)

The CFIFixup pass assumes a function prologue is contained in a single
basic block. This assumption is broken with upcoming support for stack
probing (`-fstack-clash-protection`) in AArch64 - the emitted probing
sequence in a prologue may contain loops, i.e. more than one basic
block. The generated CFG is not arbitrary though:
 * CFI instructions are outside of any loops
* for any two CFI instructions of the function prologue one dominates
and is post-dominated by the other

Thus, for the prologue CFI instructions, if one is executed then all are
executed, there is a total order of executions, and the last instruction
in that order can be considered the end of the prologoue for the purpose
of inserting the initial `.cfi_remember_state` directive.

That last instruction is found by finding the first block in the
post-order traversal which contains prologue CFI instructions.