This introduces a new constant kNeedS128RegPair, which is set for
architecture where a pair of FP registers (DoubleRegister) is mapped to
single Simd128Register (ARM).

In this case, a new RegClass, kFpRegPair, is defined to represent
LiftoffRegister of this type. kFpRegPair will be kNoReg on all other
architectures.

We add 1 more bit to the encoding on ARM, so now the top bit is set for
kFpRegPair, the second most top bit is set for kGpRegPair. When the top
bit is set, we encode the FP register code into the bottom bits of
code_. Note that this is directly encoded, i.e. not added to kMaxGpRegCode,
so we can save an add/subtract when converting to/from DoubleRegister.
We only need to store 1 FP register, the low register, since the high
other register is implicity the next register. Note that the stored
register is *always* an even-numbered register.

Bug: v8:9909
Change-Id: I78d603c9938c3d0add9bd3ca77ddebbfa7abbc05
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1973276
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65508}