Remove redundant zero-initializations for long-lifetime structs. (dotnet/coreclr...

Remove redundant zero-initializations for long-lifetime structs. (dotnet/coreclr#20753)

When compInitMem is true long-lifetime structs (i.e., the ones with lvIsTemp set to false)
are zero-initialized in the prolog: https://github.com/dotnet/coreclr/blob/dotnet/coreclr@c8a63947382b0db428db602238199ca81badbe8e/src/jit/codegencommon.cpp#L4765

Therefore, these structs don't need an explicit zero-initialization in blocks that are not in a loop.

Commit migrated from https://github.com/dotnet/coreclr/commit/dc028d9893915c4c1bd62daf5e12325c15334154