[ImplicitNullChecks] Check for rewrite of register used in 'test' instruction

The following code pattern:

       mov %rax, %rcx
       test %rax, %rax
       %rax = ....
       je  throw_npe
       mov(%rcx), %r9
       mov(%rax), %r10

gets transformed into the following incorrect code after implicit null check pass:
        mov %rax, %rcx
       %rax = ....
       faulting_load_op("movl (%rax), %r10", throw_npe)
       mov(%rcx), %r9

For implicit null check pass, if the register that is checked for null value (ie, the register used in the 'test' instruction) is written into before the condition jump, we should avoid doing the optimization.

Patch by Surya Kumari Jangala!

Differential Revision: https://reviews.llvm.org/D48627
Reviewed By: skatkov

llvm-svn: 336241