Add option `use-fn-table-in-decode-to-mcinst` to use a table of function
pointers instead of a switch case in the generated `decodeToMCInst`
function.

When the number of switch cases in this function is large, the generated
code takes a long time to compile in release builds. Using a table of
function pointers instead improves the compile time significantly (~3x
speedup in compiling the code in a downstream target). This option will
allow targets to opt into this mode if they desire for better build
times.

Tested with `check-llvm-mc` with the option enabled by default.