x86/Intel: allow MASM representation of embedded rounding / SAE

x86/Intel: allow MASM representation of embedded rounding / SAE

MASM doesn't support the separate operand form; the modifier belongs
after the instruction instead. Accept this form alongside the original
(now legacy) one. Short of having access to a MASM version to actually
check in how far "after the instruction" is a precise statement in their
documentation, allow both that and the SDM mandated form where the
modifier is on the last register operand (with a possible immediate
operand following).

Sadly the split out function, at least for the time being, needs to cast
away constness at some point, as the two callers disagree in this
regard.

Adjust some, but not all of the testcases.