x64: Lower fcopysign, ceil, floor, nearest, and trunc in ISLE

[elliottt]

Add tests for fcopysign, ceil, floor, nearest, and trunc, and lower all of those instructions in ISLE.

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[cfallin]

r+ on new commits; will approve once earlier PRs merge and this is unstacked.

Commentary below on rule ordering but I don't think we need to do anything about it now, given context!

[cfallin]

Here we're implicitly relying on the rule-ordering heuristic again (that pesky #4717!): this rule doesn't do anything to guard on not SSE4.1 and it's not clear to me that the sequence of PatternInsts is "less specific" and shadowed by the above rule without going through the compilation in my head... let's leave this for now though as we are likely to resolve #4717 soon.