change checking for auto-init metadata to use `equalsStr` instead of casing MDOperand nodes.

Since `MD_annotation` metadata now supports having mutliple strings in the annotation node. casing Operand to string directly will cause a crash. When checking if `MDOperand` equals str you can use `equalsStr` method.

Reviewed By: serge-sans-paille

Differential Revision: https://reviews.llvm.org/D152372