[PATCH] Fix PR17168 (DAG scheduler inserts DBG_VALUE before PHI with fast-isel)

PR17168 describes a test case that fails when compiling for debug with
fast-isel.  Investigation showed that the test was failing because a DBG_VALUE
machine instruction was placed prior to a PHI.

For this problem to occur requires the following:
 * Compile for debug
 * Compile with fast-isel
 * In a block B, fast-isel must partially succeed before punting to DAG-isel
 * B must start with a PHI
 * The first unhandled node in the DAG must not generate a machine instruction
 * A debug value with an order less than that of that first node exists

When all of these circumstances apply, the existing test that an instruction
was not inserted won't fire.  Currently it tests whether the block is empty,
or whether the last instruction generated is a phi.  When fast-isel has
partially succeeded, the last instruction generated will not be a phi.
Instead, we need to check whether the current insert position is immediately
following a phi.  This patch adds that check, and adds the test case from the
PR as a regression test.