MachineSSAUpdater: insert IMPLICIT_DEF at top of basic block

Summary:
When getValueInMiddleOfBlock happens to be called for a basic block
that has no incoming value at all, an IMPLICIT_DEF is inserted in that
block via GetValueAtEndOfBlockInternal. This IMPLICIT_DEF must be at
the top of its basic block or it will likely not reach the use that
the caller intends to insert.

Issue: https://github.com/GPUOpen-Drivers/llpc/issues/204

Reviewers: arsenm, rampitec

Subscribers: jvesely, wdng, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68183

llvm-svn: 374040