[MCA] Minor refactoring of method DefaultResourceStrategy::select. NFCI

Common code used by the default resource strategy to select pipeline resources
has been moved to an helper function.

The new selection logic has been slightly rewritten to get rid of a redundant
zero check on the `ReadyMask` value. Before this patch, method select internally
called function `PowerOf2Floor` to compute the next ready pipeline resource.
However, `PowerOf2Floor` forces an implicit (redundant) zero check on the input
value. By construction, `ReadyMask` can never be zero. This patch replaces the
call to `PowerOf2Floor` with an equivalent block of code which avoids the
redundant zero check. This gives a minor 3-3.5% speedup on a release build.

No functional change intended.

llvm-svn: 350218