Support deoptimization only to set vreg

This CL removes the old API that was used to update vreg. We now rely
exclusively on deoptimization to change the value of a vreg (mainly
from the debugger). This allows to have only one mechanism working
with both Quick and Optimizing compilers. It also remove run-test
455-set-vreg which was the only user of the old API.

This CL also renames StackVisitor::SetVReg[Pair]FromDebugger to
StackVisitor::SetVReg[Pair] to reflect that change.

Bug: 25428216
Change-Id: Ib09510185280dbd0f6d02c52549ae9671c187e32