[x86] fix cost of SINT_TO_FP for i32 --> float (PR21356, PR28434)

This is "cvtdq2ps" which does not appear to be particularly slow on any CPU
according to Agner's tables. Choosing "5" as a cost here as suggested in:
https://llvm.org/bugs/show_bug.cgi?id=21356
...but it seems very conservative given that the instruction is fully pipelined,
and I think these costs are supposed to model throughput.

Note that related costs are also most likely too high, but this fixes PR21356
and partly fixes PR28434.

llvm-svn: 274658