Fix missing flags for IND(str_handle)

[EgorBo]

Fixes#75562
Normally, we set GTF_IND_INVARIANT for string literals when we create IND nodes representing them, but after #49576GTF_ICON_STR_HDL may be created without GT_IND first, so if it appears later - update its flags.

Also, fix eeGetCPString (used for e.g. DOTNET_JitDisasm) to handle frozen strings

cc @AndyAyersMS

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Issue Details

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Fixes #75562
Normally, we set GTF_IND_INVARIANT for string literals when we create IND nodes representing them, but after #49576GTF_ICON_STR_HDL may be created without GT_IND first, so if it appears later - update its flags.

Also, fix eeGetCPString (used for e.g. DOTNET_JitDisasm) to handle frozen strings

cc @AndyAyersMS

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