Adjust JitCodeCache::TransitionToDebuggable.

Only clear the precompiled flag if the method has it. Also added some
comments.

Bug: 144947842
Test: atest CtsJvmtiRunTest1982HostTestCases
Test: 689-zygote-jit-deopt

(cherry-picked from commit fc47d6bf37703ba58960a4b25723263a160c5555)

Merged-In: I8b2856823f359076c41240f463902eb57c1f7603
Change-Id: I8b2856823f359076c41240f463902eb57c1f7603
1 file changed