[CostModel][X86] i8/i16 sitofp/uitofp are sext/zext to i32 for sitofp

[CostModel][X86] i8/i16 sitofp/uitofp are sext/zext to i32 for sitofp

Provide a generic fallback that extends sub-i32 scalars before using the existing sitofp instructions.

These numbers can be tweaked for specific sse levels, but we should get the default handling in place first.

We get the extension for free for non-vector loads.