Track dependently-promoted fields when struct local is no longer referenced.

[erozenfeld]

This is a follow-up to #36918. It addresses one of the examples in #1007
where we remove a struct zero initialization but fail to clean up a dead
field assignment.

The change is not to mark a dependently promoted field as untracked
if we know that the struct local is no longer referenced.

I also addressed a couple of late cosmetic review comments from #36918.

No diffs in framework and benchmarks.

[erozenfeld]

Example from #1007:

Example 4:

using System.Runtime.InteropServices;

public class C {
    [StructLayout(LayoutKind.Sequential, Size = 24)]
    public struct Buffer
    {
        public ulong Field1;
        public ulong Field2;
        public uint Field3;
    }
    
    public void M() {
        var buf = new Buffer();
        buf.Field1 = 2;
    }
}

Diffs after #36918 (optimization to remove zero inits):

G_M25775_IG01:
       4883EC18             sub      rsp, 24
-      C5F877               vzeroupper
       C5D857E4             vxorps   xmm4, xmm4
       C5F97F2424           vmovdqa  xmmword ptr [rsp], xmm4
       33C0                 xor      rax, rax
       4889442410           mov      qword ptr [rsp+10H], rax
                                                ;; bbWeight=1    PerfScore 3.83
G_M25775_IG02:
-      33C0                 xor      eax, eax
-      C5F857C0             vxorps   xmm0, xmm0
-      C5FA7F0424           vmovdqu  xmmword ptr [rsp], xmm0
-      4889442410           mov      qword ptr [rsp+10H], rax
       48C7042402000000     mov      qword ptr [rsp], 2
G_M25775_IG03:
       4883C418             add      rsp, 24
       C3                   ret

-; Total bytes of code 52, prolog size 23, PerfScore 14.27, (MethodHash=0c929b50) for method C:M()
+; Total bytes of code 33, prolog size 20, PerfScore 8.58, (MethodHash=0c929b50) for method C:M()

Diffs after this change:

G_M25775_IG01:
-      4883EC18             sub      rsp, 24
-      C5D857E4             vxorps   xmm4, xmm4
-      C5F97F2424           vmovdqa  xmmword ptr [rsp], xmm4
-      33C0                 xor      rax, rax
-      4889442410           mov      qword ptr [rsp+10H], rax
G_M25775_IG02:
-      48C7042402000000     mov      qword ptr [rsp], 2
G_M25775_IG03:
-      4883C418             add      rsp, 24
       C3                   ret

-; Total bytes of code 33, prolog size 20, PerfScore 8.58, (MethodHash=0c929b50) for method C:M()
+; Total bytes of code 1, prolog size 0, PerfScore 1.10, (MethodHash=0c929b50) for method C:M()

[erozenfeld]

@dotnet/jit-contrib @CarolEidt PTAL

[erozenfeld]

I pushed a change that fixes Linux arm test failures.
@CarolEidt @dotnet/jit-contrib PTAL

[CarolEidt]

LGTM, with just one question for curiosity's sake.

[CarolEidt]

If the field is dependently promoted (i.e. the whole struct is forced to live on the stack), then do we still need to mark them as lvLiveInOutOfHndlr since they'll always be defined/used to/from the stack?

[erozenfeld]

With this change we may enregister a field of a dependently promoted struct if the struct is no longer used. See the change in lclvars.cpp: we don't set lvaSetVarDoNotEnregister on such fields.