[ARM] Hoist fabs/fneg above a conversion to float.

This is especially visible in softfp mode, for example in the implementation of libm fabs/fneg functions. If we have:

%1 = vmovdrr r0, r1
%2 = fabs %1

then move the fabs before the vmovdrr:

%1 = and r1, #0x7FFFFFFF
%2 = vmovdrr r0, r1

This is never a lose, and could be a serious win because the vmovdrr may be followed by a vmovrrd, which would enable us to remove the conversion into FPRs completely.

We already do this for f32, but not for f64. Tests are added for both.

llvm-svn: 246360