Fix access to long/double stack values from debugger

Long and double values live in a pair of DEX registers. When we compile DEX
code with the Quick compiler, a DEX register either lives in the stack or is
promoted to a physical register. In the case of a pair of DEX registers, the
Quick compiler assumes both registers live in the same "area": both live in
the stack or both are promoted to physical registers.

From the debugger, we used to access these values by reading/writing each DEX
register separately. However, this does not work when only one DEX register of
a pair is promoted and the other lives in the stack. In this case, the compiled
code reads from/writes to the stack only.

To fix that, the debugger must follow the same rule than the Quick compiler: if
both DEX registers are promoted, read/write them from/to physical registers,
otherwise read/write them from/to the stack. We add StackVisitor:GetVRegPair and
StackVisitor:SetVRegPair for this purpose.

We also follow the same rule when deoptimizing. However we need to do that only
when we know two consecutive DEX registers are part of a pair (long or double).
We know that thanks to the verifier.

Bug: 15527793
Change-Id: I04812285ff26ef0129f39792a1cf776f3591ca2d