[BOLT] Refactor fixBranches() (#73752)

Simplify code in fixBranches(). Mostly NFC, accept the x86-specific
check for code fragments now takes into account presence of more than
two fragments. Should only matter when we split code into multiple
fragments and can run fixBranches() more than once.

Also, don't replace a branch target with the same one, as such operation
may allocate memory for extra MCSymbolRefExpr.