Fix Block::eraseArgument when block arg is also a successor operand.

Summary:
This could trigger an assertion due to the block argument being used by
this block's own successor operands.

Reviewers: rriddle!

Subscribers: mehdi_amini, jpienaar, burmako, shauheen, antiagainst, nicolasvasilache, arpith-jacob, mgester, lucyrfox, liufengdb, Joonsoo, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D74583