Some GT_BLK/GT_OBJ related cleanup (dotnet/coreclr#27053)

Some GT_BLK/GT_OBJ related cleanup (dotnet/coreclr#27053)

* Display class layout name in dumps

* Delete unused gtNewBlkOpNode size parameter

* gtNewObjNode always returns a GenTreeObj

* Can't have GTF_VAR_DEATH on a GT_OBJ node

* Delete unnecessary uses of gtNewCpObjNode

* Delete unnecessary uses of gtNewBlockVal

* Delete fgMorphBlkToInd

Modifying nodes in place is the norm in JIT, there's no need for a function to abstract that.

* Delete unused RewriteSIMDOperand keepBlk parameter

* Cleanup Rationalizer::RewriteSIMDOperand

Commit migrated from https://github.com/dotnet/coreclr/commit/8f9eb95bb1df95911c121fdafd65c3fe8d490a75