[X86][FastIsel] Avoid introducing legacy SSE instructions if the target has AVX.

This patch teaches X86FastISel how to select AVX instructions for scalar
float/double convert operations.

Before this patch, X86FastISel always selected legacy SSE instructions
for FPExt (from float to double) and FPTrunc (from double to float).

For example:
\code
  define double @foo(float %f) {
    %conv = fpext float %f to double
    ret double %conv
  }
\end code

Before (with -mattr=+avx -fast-isel) X86FastIsel selected a CVTSS2SDrr which is
legacy SSE:
  cvtss2sd %xmm0, %xmm0

With this patch, X86FastIsel selects a VCVTSS2SDrr instead:
  vcvtss2sd %xmm0, %xmm0, %xmm0

Added test fast-isel-fptrunc-fpext.ll to check both the register-register and
the register-memory float/double conversion variants.

Differential Revision: http://reviews.llvm.org/D7438

llvm-svn: 228682