This commit moves the fusion decoder to both decode and rename stage.

In the decode stage, fusion decoder determines whether the instruction
pairs can be fused. Valid bits of decode are not affected by fusion
decoder. This should fix the timing issues of rename.valid.

In the rename stage, some fields are updated according the result of
fusion decoder. This will bring a minor timing path to both valid and
other fields in uop in the rename stage. However, since freelist and
rat have worse timing. This should not cause timing issues.