[X86] Don't allow combineSIntToFP to create v2i32 vectors after type legalization.

If we're after type legalization we should only be trying to turn
v2i64 into v2i32. So bitcast to v4i32, shuffle the even elements
together. Then use X86ISD::CVTSI2P. The alternative is to leave
the v2i64 type alone and let it scalarized. Hopefully keeping
it packed is better.

Fixes PR42905.

llvm-svn: 368091