Fix not correct imm operand assertion for SUB32ri in X86CondBrFolding::analyzeCompare

Summary:
When doing X86CondBrFolding::analyzeCompare, it will meet the SUB32ri instruction as below to use the global address for its operand,
  %733:gr32 = SUB32ri %62:gr32(tied-def 0), @img2buf_normal, implicit-def $eflags
  JNE_1 %bb.41, implicit $eflags

so the assertion "assert(MI.getOperand(ValueIndex).isImm() && "Expecting Imm operand")" is not correct and change the assert to if make X86CondBrFolding::analyzeCompare return false as not finding the compare for this

Patch by Jianping Chen

Reviewers: smaslov, LuoYuanke, liutianle, Jianping

Reviewed By: Jianping

Subscribers: lebedev.ri, llvm-commits

Differential Revision: https://reviews.llvm.org/D54250

llvm-svn: 348853