[BPF] Reset machine register kill mark in BPFMISimplifyPatchable

When LLVM is build with `LLVM_ENABLE_EXPENSIVE_CHECKS=ON` option
the following C code snippet:

    struct t {
      unsigned long a;
    } __attribute__((preserve_access_index));

    void foo(volatile struct t *t, volatile unsigned long *p) {
      *p = t->a;
      *p = t->a;
    }

Causes an assertion:

    $ clang -g -O2 -c --target=bpf -mcpu=v2 t2.c -o /dev/null

    # After BPF PreEmit SimplifyPatchable
    # Machine code for function foo: IsSSA, TracksLiveness
    Function Live Ins: $r1 in %0, $r2 in %1

    bb.0.entry:
      liveins: $r1, $r2
      DBG_VALUE $r1, $noreg, !"t", !DIExpression()
      DBG_VALUE $r2, $noreg, !"p", !DIExpression()
      %1:gpr = COPY $r2
      DBG_VALUE %1:gpr, $noreg, !"p", !DIExpression()
      %0:gpr = COPY $r1
      DBG_VALUE %0:gpr, $noreg, !"t", !DIExpression()
      %2:gpr = LD_imm64 @"llvm.t:0:0$0:0"
      %4:gpr = ADD_rr %0:gpr(tied-def 0), killed %2:gpr
      %5:gpr = CORE_LD 344, %0:gpr, @"llvm.t:0:0$0:0"
      STD killed %5:gpr, %1:gpr, 0
      %7:gpr = ADD_rr %0:gpr(tied-def 0), killed %2:gpr
      %8:gpr = CORE_LD 344, %0:gpr, @"llvm.t:0:0$0:0"
      STD killed %8:gpr, %1:gpr, 0
      RET

    # End machine code for function foo.

    *** Bad machine code: Using a killed virtual register ***
    - function:    foo
    - basic block: %bb.0 entry (0x6210000e6690)
    - instruction: %7:gpr = ADD_rr %0:gpr(tied-def 0), killed %2:gpr
    - operand 2:   killed %2:gpr

This happens because of the way
BPFMISimplifyPatchable::processDstReg() updates second operand of the
`ADD_rr` instruction. Code before `BPFMISimplifyPatchable`:

    .-> %2:gpr = LD_imm64 @"llvm.t:0:0$0:0"
    |
    |`----------------.
    |   %3:gpr = LDD %2:gpr, 0
    |   %4:gpr = ADD_rr %0:gpr(tied-def 0), killed %3:gpr <--- (1)
    |   %5:gpr = LDD killed %4:gpr, 0       ^^^^^^^^^^^^^
    |   STD killed %5:gpr, %1:gpr, 0        this is updated
     `----------------.
        %6:gpr = LDD %2:gpr, 0
        %7:gpr = ADD_rr %0:gpr(tied-def 0), killed %6:gpr <--- (2)
        %8:gpr = LDD killed %7:gpr, 0       ^^^^^^^^^^^^^
        STD killed %8:gpr, %1:gpr, 0        this is updated

Instructions (1) and (2) would be updated to:

    ADD_rr %0:gpr(tied-def 0), killed %2:gpr

The `killed` mark is inherited from machine operands `killed %3:gpr`
and `killed %6:gpr` which are updated inplace by `processDstReg()`.

This commit updates `processDstReg()` reset kill marks for updated
machine operands to keep liveness information conservatively correct.

Differential Revision: https://reviews.llvm.org/D157805