high memory usage when open single source file

[katsusan]

I have heard that gopls will try to parse or load almost everything in memory, so not sure whether it is intended or beyond the design and post here before sumbiting a issue(similar issue: #1859).

reproducing steps:

1. ctrl + shift + N to open a new window in VSCode.
2. open $GOROOT/src/cmd/compile/internal/ssa/rewritegeneric.go.
3. memory usage of gopls increased to about 2.9GB.

proof:
// open rewritegeneric.go

// open rewritegeneric.go and src/cmd/compile/main.go

[hyangah]

Given the size of the Go project, I am not too surprised by the memory usage - Gopls basically parses and loads all the type information of files in the module even when opening a single file of a big module. Gopls team has some ideas to reduce the memory usage further. cc @findleyr