coverage: Store the number of counters/expressions in function coverage info

Coverage codegen can now allocate arrays based on the number of
counters/expressions originally used by the instrumentor.

The existing query that inspects coverage statements is still used for
determining the number of counters passed to `llvm.instrprof.increment`. If
some high-numbered counters were removed by MIR optimizations, the instrumented
binary can potentially use less memory and disk space at runtime.

compiler/rustc_middle/src/mir/coverage.rs

@@ -140,4 +140,6 @@ impl Op {
 #[derive(TyEncodable, TyDecodable, Hash, HashStable, TypeFoldable, TypeVisitable)]
 pub struct FunctionCoverageInfo {
     pub function_source_hash: u64,
+    pub num_counters: usize,
+    pub num_expressions: usize,
 }