Fix too wide memory accesses for nullchecks

[EgorBo]

Fixes #64657

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Issue Details

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Fixes #64657

for now just a draft to see CI status

Author:	EgorBo
Assignees:	-
Labels:	area-CodeGen-coreclr
Milestone:	-

[EgorBo]

@BruceForstall @jakobbotsch PTAL current version.
no diffs on ARM64 as expected - all loads are 4-bytes long. A couple of 1byte diffs on x64 in cases where larger mov is used, e.g.:

struct Foo
{
    public byte b1;
    public byte b2;
}

void Test(ref Foo f)
{
    var _ = f.b2; // implicit nullcheck
}