AWS: only cache instance requirements when needed

[rrangith]

What type of PR is this?

/kind bug
/kind regression

What this PR does / why we need it:

This PR reduces the number of unneeded DescribeLaunchTemplateVersions API calls. Previously there was a PR that only looked for instance requirements when needed kubernetes#5550. However this PR kubernetes#6245 introduced a regression where now it looks for instance requirements all the time.

However in the original fix PR it was stated:

But retrieving InstanceRequirements should only be useful when the LT overrides don't already specify an InstanceType, as both are mutually exclusive. The api reference doc states:

If you specify InstanceRequirements, you can't specify InstanceType.

That mutual exclusion is already leveraged by getInstanceTypesForAsgs: we don't look for InstanceRequirements when we have a mixed instance policy specifying instance types overrides.

This PR makes it so we again only look for instance requirements if instance type overrides are not used.

This screenshot shows the huge increase in DescribeLaunchTemplateVersions API calls when we upgraded our cluster autoscaler to include the bug PR at 16:00 kubernetes#6245

Special notes for your reviewer:

Does this PR introduce a user-facing change?

AWS: Only cache instance requirements when needed

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.: