[X86][SSE] Update the cost table for integer-integer conversions on SSE2/SSE4.1.

Previously in the conversion cost table there are no entries for integer-integer
conversions on SSE2. This will result in imprecise costs for certain vectorized
operations. This patch adds those entries for SSE2 and SSE4.1. The cost numbers
are counted from the result of running llc on the new test case in this patch.


Differential revision: http://reviews.llvm.org/D15132

llvm-svn: 255315