[X86] Preserve fpexcept property when turning strict_fp_extend and...

[X86] Preserve fpexcept property when turning strict_fp_extend and strict_fp_round into stack operations.

We use the stack for X87 fp_round and for moving from SSE f32/f64 to
X87 f64/f80. Or from X87 f64/f80 to SSE f32/f64.

Note for the SSE<->X87 conversions the conversion always happens in the
X87 domain. The load/store ops in the X87 instructions are able
to signal exceptions.