[BOLT] Add REX prefix rebalancing pass

Summary:
Add a pass to rebalance the usage of REX prefixes, moving them
from the hot code path to the cold path whenever possible. To do this, we
rank the usage frequency of each register and exchange an X86 classic reg
with an extended one (which requires a REX prefix) whenever the classic
register is being used less times than the extended one. There are two
versions of this pass: regular one will only consider RBX as classic and
R12-R15 as extended registers because those are callee-saved, which means
their scope is local to the function and therefore they can be easily
interchanged within the function without further consequences. The
aggressive version relies on liveness analysis to detect if the value of
a register is being used as a caller-saved value (written to without
being read first), which also is eligible for reallocation. However, it
showed limited results and is not the default option because it is
expensive.

Currently, this pass does not update debug info. This means that if a
substitution is made, the AT_LOCATION of a variable inside a function may
be outdated and GDB will display the wrong value if you ask it to print
the value of the affected variable. Updating DWARF involves a painful
task of writing a new DWARF expression parser/writer similar to the one
we already have for CFI expressions. I'll defer the task of writing this
until we determine this optimization is enabled in production. So far,
it is experimental to be combined with other optimizations to help us
find a new set of optimizations that is beneficial.

(cherry picked from FBD6476659)