[Win64] Handle FP arguments more gracefully under -mno-sse

Pass small FP values in GPRs or stack memory according the the normal
convention. This is what gcc -mno-sse does on Win64.

I adjusted the conditions under which we emit an error to check if the
argument or return value would be passed in an XMM register when SSE is
disabled. This has a side effect of no longer emitting an error for FP
arguments marked 'inreg' when targetting x86 with SSE disabled. Our
calling convention logic was already assigning it to FP0/FP1, and then
we emitted this error. That seems unnecessary, we can ignore 'inreg' and
compile it without SSE.

Reviewers: jyknight, aemerson

Differential Revision: https://reviews.llvm.org/D70465