1. grow/shrink frame size in the pro/epilogue. Note: though we removed
push/pop for unused callee saved registers, we still reserve frame space
for them. Because we generate spill on frame before we know exactly how
large a space is. The solution is to make spill locations patchable.
2. control flow will combine branches in switch instruction if they all
target the same destination.

shl, shr, xor, and. Need to deal with large constant which cannot be fit
as instruction immediate (32bits)

now)

Added supports for all types.

Support Const NULL

Support conversions, the TAILCALL and NEW instructions

now