Translator: Cap the constant slot size to avoid slot explosion

Some large functions have a lot of constants than we expected. For
example, `SwiftSyntax.SyntaxRewriter.dispatchVisit(_:)` is over 70kb in
wasm binary size and has 1000+ unique constants, so the constant slot
size was over 1000. This caused a call to the function to consume a lot
of stack space, even though the function only accesses a few of them in
a single function call. This patch caps the constant slot size to 128
to avoid slot explosion.

Author: kateinoigakukun

Sources/WasmKit/Translator.swift

@@ -216,7 +216,9 @@ struct StackLayout {
         self.frameHeader = FrameHeaderLayout(type: type)
         self.numberOfLocals = numberOfLocals
         // The number of constant slots is determined by the code size
-        self.constantSlotSize = max(codeSize / 20, 4)
+        // This is a heuristic value to balance the fast access to constants
+        // and the size of stack frame. Cap the slot size to avoid size explosion.
+        self.constantSlotSize = min(max(codeSize / 20, 4), 128)
         let (maxSlots, overflow) = self.constantSlotSize.addingReportingOverflow(numberOfLocals)
         guard !overflow, maxSlots < VReg.max else {
             throw TranslationError("The number of constant slots overflows")