[X86] Simplify the ReplaceNodeResults code for X86ISD::AVG.

This code seemed to try to widen to 128, 256, or 512 bit vectors, but we only create X86ISD::AVG with a power of 2 number of elements. This means the only nodes that need to be legalized are less than 128-bits and need to be widened up to 128 bits.

llvm-svn: 326064