[Coroutines] Remove corresponding phi values when apply simplifyTerminatorLeadingToRet

Summary:
In addMustTailToCoroResumes, we set musttail on those resume instructions that are followed by a ret instruction. This is done by simplifyTerminatorLeadingToRet which replace a sequence of branches leading to a ret with a clone of the ret.

However it forgets to remove corresponding PHI values that come from basic block of replaced branch, and may cause jumpthreading pass hangs (https://bugs.llvm.org/show_bug.cgi?id=43720)

This patch fix this issue

Test Plan:
cppcoro library with O3+flto
check-llvm

Reviewers: modocache, GorNishanov, lewissbaker

Reviewed By: modocache

Subscribers: mehdi_amini, EricWF, hiraditya, dexonsmith, jfb, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71826

Patch by junparser (JunMa)!