[PPC64] Fix PR20071 (fctiduz generated for targets lacking that instruction)

PR20071 identifies a problem in PowerPC's fast-isel implementation for
floating-point conversion to integer.  The fctiduz instruction was added in
Power ISA 2.06 (i.e., Power7 and later).  However, this instruction is being
generated regardless of which 64-bit PowerPC target is selected.

The intent is for fast-isel to punt to DAG selection when this instruction is
not available.  This patch implements that change.  For testing purposes, the
existing fast-isel-conversion.ll test adds a RUN line for -mcpu=970 and tests
for the expected code generation.  Additionally, the existing test
fast-isel-conversion-p5.ll was found to be incorrectly expecting the
unavailable instruction to be generated.  I've removed these test variants
since we have adequate coverage in fast-isel-conversion.ll.