[Relax][Bugfix] FCallPacked not checked in CodegenVMTIR

[Lunderberg]

Prior to this commit, an operator's FCallPacked attribute, used to specify a 1:1 mapping between a relax operator and a PackedFunc that implements it, was only checked in CodegenVM. Any operator with FCallPacked would raise an error when compiled using CodegenVMTIR.

This commit removes the FCallPacked handling from CodegenVM altogether, and instead checks for this attribute as part of LegalizeOps. This provides the same functionality across both backends.

[Lunderberg]

@tvm-bot rerun

[tqchen]

@Lunderberg need rebase

[Lunderberg]

Thank you, and rebased to resolve conflicts. (And again to resolve a conflict that wasn't flagged by git.)