cmd/compile/internal/inline: debug flag to alter score adjustments

Add a debugging flag "-d=inlscoreadj" intended to support running
experiments in which the inliner uses different score adjustment
values for specific heuristics. The flag argument is a series of
clauses separated by the "/" char where each clause takes the form
"adjK:valK". For example, in this build

  go build -gcflags=-d=inlscoreadj=inLoopAdj:10/returnFeedsConstToIfAdj:-99

the "in loop" score adjustments would be reset to a value of 15 (effectively
penalizing calls in loops) adn the "return feeds constant to foldable if/switch"
score adjustment would be boosted from -15 to -99.

Change-Id: Ibd1ee334684af5992466556a69baa6dfefb246b3
Reviewed-on: https://go-review.googlesource.com/c/go/+/532116
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/cmd/compile/internal/base/debug.go

@@ -23,6 +23,8 @@ type DebugFlags struct {
 	DisableNil            int    `help:"disable nil checks" concurrent:"ok"`
 	DumpInlFuncProps      string `help:"dump function properties from inl heuristics to specified file"`
 	DumpInlCallSiteScores int    `help:"dump scored callsites during inlining"`
+	InlScoreAdj           string `help:"set inliner score adjustments (ex: -d=inlscoreadj=panicPathAdj:10/passConstToNestedIfAdj:-90)"`
+	InlBudgetSlack        int    `help:"amount to expand the initial inline budget when new inliner enabled. Defaults to 80 if option not set." concurrent:"ok"`
 	DumpPtrs              int    `help:"show Node pointers values in dump output"`
 	DwarfInl              int    `help:"print information about DWARF inlined function creation"`
 	EscapeMutationsCalls  int    `help:"print extra escape analysis diagnostics about mutations and calls" concurrent:"ok"`