[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;

[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

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