[MLIR][NFC] rename ConvertStandardToLLVM, ConvertLoopToStandard to drop Convert prefix

This is in line with the convention agreed on https://llvm.discourse.group/t/rfc-canonical-file-paths-to-dialects/621



Signed-off-by: Uday Bondhugula <uday@polymagelabs.com>

Differential Revision: https://reviews.llvm.org/D76583