Eliminate redundant register swapping in JIT

In the current register allocation of JIT, we save the host registers
before using to keep the consistency of the content between the vm and
the host register, even it had not been changed by any other operation
after loaded from the stack. This generates lots of redundant `store`
related instructions on the host and reduces the performance of vm.

In this patch, we introduce the `dirty` flag to record the status of
the host register. Once the content of the host register is changed by
an operation, the flag is set to 1. The register swapping will only be
invoked when the register is dirty in next allocation.

The performance improvement after this patch is shown below:

* x86-64
| Metric    | Original | Improved | SpeedUp |
|-----------+----------+----------+---------|
| dhrystone |  0.623 s |  0.463 s |  +25.6% |
| miniz     |  3.437 s |  2.837 s |  +17.5% |
| primes    |  4.435 s |  3.205 s |  +27.7% |
| sha512    |  3.212 s |  3.024 s |   +5.9% |
| aes       |  1.708 s |  1.545 s |   +9.5% |
| qsort     |  5.236 s |  5.219 s |   +0.3% |

* aarch64
| Metric    | Original | Improved | SpeedUp |
|-----------+----------+----------+---------|
| dhrystone |  1.003 s |  0.897 s |  +10.6% |
| miniz     |  4.312 s |  4.107 s |   +4.8% |
| primes    |  9.605 s |  8.879 s |   +7.6% |
| sha512    |  5.902 s |  5.796 s |   +1.8% |
| aes       |  2.932 s |  2.891 s |   +1.4% |
| qsort     |  6.123 s |  5.988 s |   +2.2% |

Close #330

Author: vacantron