cmd/compile: add size cache to StdSizes to avoid exponential compile time

[hesam-oxe]

Added a sizeCache map with sync.RWMutex to StdSizes to memoize
Sizeof results for deeply nested generic types.

Problem

When compiling deeply nested generic instantiations like:

type T[P any] struct { _,_,_,_,_,_,_,_ P }
type U T[T[T[T[T[T[T[T[T[T[int]]]]]]]]]]

StdSizes.Sizeof was called recursively without caching, resulting in
exponential time (8^N where N is nesting depth).

Root Cause

Sizeof for *Struct and *Array types recursively computes sizes
of their elements/fields. For nested generic types, the same underlying
type is recomputed millions of times.

Fix

• Added sizeCache map[Type]int64 and sync.RWMutex to StdSizes
• Sizeof checks the cache before computing, and stores results after
• Lazy initialization of the cache map

Impact

• Compilation time for deeply nested generics drops from exponential to linear
• No API change — cache is transparent to callers
• Thread-safe via RWMutex

Fixes #78342

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[hesam-oxe]

/verify-cla

[hesam-oxe]

@google-cla I've signed the Individual CLA with email
chngyzkhanwhsht@gmail.com. Can you please re-check?