Use LclFld for full-width cpblk of different types

Use LclFld for full-width cpblk of different types

This issue arose because the source tree was a struct with a single double field, and the destination was a struct with a single long field. Copy prop replaced the lclVar in the source, which was under a `IND(ADDR)` with a lclVar that was not address taken, although it was marked `GTF_DONT_CSE`. Replacing the src tree with a `GT_LCL_FLD` addresses this issue.

Also, ensure that the target is marked `DoNotEnregister` if it isn't referenced as the same size/type.

Fix dotnet/coreclr#21080
Fix dotnet/coreclr#21064

Commit migrated from https://github.com/dotnet/coreclr/commit/0c08dc461f4dd7ac418a475264129d2c57536955