a jump that directly is directly followed by a label of its target
can be removed.

1. asm call do not use all argument registers (otherwise it will keep
them alive)
2. spilling a register that is used and defined in one instruction will
result in creating one new temporary, instead of two
3. spilling now deals with floating point
4. SELECT with int8 is implemented using conditional jump (cmov cannot
take reg8)
5. postcall convention now deals correctly with fp return values
6. reg alloc conservative() was wrong in a few commits ago, fixed it
7. in liveness analysis, when finding a move between a temp and a
register, find the color for the register (such as RAX for EAX)

currently mov constant as imm to a GPR, then mov from GPR
to xmm. A better approach should be put the immediate in memroy,
and load it

when we add interefence between %a and %edi, we also add %a with %rdi
because we cannot assign rdi to a

wip: refactor performance comparison code. Need floating point support to embed time measurement code in RPython side

for movl %edi %edi (where two operands are actually %edi %rdi), we will
do a check, and ensure it get removed

changed a condition on coealescing. Fixed a bug in spilling rewrite.
Currently test_extern_Func is broken