Fix dump of GC vars at call sites

[BruceForstall]

GC vars are handled specially at call sites. In particular,
kills happen at the call site address, meaning before the
call. They JIT dump was displaying the kill after the call,
which is confusing, and different from all the other cases
of displaying GC refs. Fix this so the kills are properly
displayed before the call site.

Example diff:

call     System.Collections.Immutable.ImmutableArray:ToImmutableArray(System.Collections.Generic.IEnumerable`1[__Canon]):System.Collections.Immutable.ImmutableArray`1[__Canon]
    ; gcrRegs -[rdx r14]
    ; GC ptr vars -{V21}
    ; gcr arg pop 0

=>

    ; GC ptr vars -{V21}
call     System.Collections.Immutable.ImmutableArray:ToImmutableArray(System.Collections.Generic.IEnumerable`1[__Canon]):System.Collections.Immutable.ImmutableArray`1[__Canon]
    ; gcrRegs -[rdx r14]
    ; gcr arg pop 0

[BruceForstall]

@dotnet/jit-contrib

I need to merge to the post-src-removal change.

[sandreenko]

LGTM.
I was not sure so I checked how Allocating Registers is showing it, it actually shows that the local var is dying before the call and then we have a live fixedReg interval for the register

DefList: {  }
N031 (  3,  2) [000007] ------------              *  LCL_VAR   int    V01 loc0         u:3 NA (last use) REG NA $140

DefList: {  }
N033 (???,???) [000025] ------------              *  PUTARG_REG int    REG rcx
<RefPosition #7   @33  RefTypeFixedReg <Reg:rcx>  BB03 regmask=[rcx] minReg=1>
<RefPosition #8   @33  RefTypeUse <Ivl:1 V01> LCL_VAR BB03 regmask=[rcx] minReg=1 last fixed> <- last use for V01, it is dead after it, happens before the call
Interval  2: int RefPositions {} physReg:NA Preferences=[allInt]
<RefPosition #9   @34  RefTypeFixedReg <Reg:rcx>  BB03 regmask=[rcx] minReg=1>
<RefPosition #10  @34  RefTypeDef <Ivl:2> PUTARG_REG BB03 regmask=[rcx] minReg=1 fixed>

DefList: { N033.t25. PUTARG_REG }
N035 ( 17,  8) [000008] --CXG-------              *  CALL      void   UnitTest1.DoSomething REG NA $VN.Void
<RefPosition #11  @35  RefTypeFixedReg <Reg:rcx>  BB03 regmask=[rcx] minReg=1>
<RefPosition #12  @35  RefTypeUse <Ivl:2>  BB03 regmask=[rcx] minReg=1 last fixed>

33.#8  V1   Use *  Copy  rcx  |V1 a|V1 a|    |    |    |    |    |    |    | <- V1 is dead now
34.#9  rcx  Fixd   Keep  rcx  |    |    |    |    |    |    |    |    |    |
34.#10 I2   Def    Alloc rcx  |    |I2 a|    |    |    |    |    |    |    |
35.#11 rcx  Fixd   Keep  rcx  |    |I2 a|    |    |    |    |    |    |    | <- first call refPosition
35.#12 I2   Use *  Keep  rcx  |    |I2 a|    |    |    |    |    |    |    |

so it makes sense to show it dying before the call.

[BruceForstall]

fwiw, SOS !u -gcinfo also shows it dying before the call

[benaadams]

Needs git rebase upstream/master rebasing for the additional /src/ removal in path