[CodeGen] Support (and default to) expanding READCYCLECOUNTER to 0.

[CodeGen] Support (and default to) expanding READCYCLECOUNTER to 0.

For targets that didn't support this, this will let us respect the
langref instead of failing to select.

Note that we don't need to change the 32-bit x86/PPC lowerings (to
account for the result type/# difference) because they're both
custom and bypass type legalization.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@246258 91177308-0d34-0410-b5e6-96231b3b80d8