Where possible, use SvIV instead of SvIVX, SvNV instead of SvNVX,

Where possible, use SvIV instead of SvIVX, SvNV instead of SvNVX,
SvUV instead of SvUVX, and SvPV* variants instead of SvPVX*.
Document that the non-x variants are preferable whenever the expression
has no side effects. (Compilers perform common subexression
elimination). Likewise SvREFCNT_inc simple variants are valid for all
cases apart from expressions with side effects.

p4raw-id: //depot/perl@31010