[InstrInfo] Refactor foldOperandImpl to thread through InsertPt. NFC

[InstrInfo] Refactor foldOperandImpl to thread through InsertPt. NFC

Summary:
This was a longstanding FIXME and is a necessary precursor to cases
where foldOperandImpl may have to create more than one instruction
(e.g. to constrain a register class). This is the split out NFC changes from
D6262.

Reviewers: pete, ributzka, uweigand, mcrosier

Reviewed By: mcrosier

Subscribers: mcrosier, ted, llvm-commits

Differential Revision: http://reviews.llvm.org/D10174

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@239336 91177308-0d34-0410-b5e6-96231b3b80d8