[Transforms][GlobalSRA] huge array causes long compilation time and huge memory usage.

Summary:
For artificial cases (huge array, few usages), Global SRA optimization creates
a lot of redundant data. It creates an instance of GlobalVariable for each array
element. For huge array, that means huge compilation time and huge memory usage.
Following example compiles for 10 minutes and requires 40GB of memory.

namespace {
  char LargeBuffer[64 * 1024 * 1024];
}

int main ( void ) {

    LargeBuffer[0] = 0;

    printf("\n ");

    return LargeBuffer[0] == 0;
}

The fix is to avoid Global SRA for large arrays.

Reviewers: craig.topper, rnk, efriedma, fhahn

Reviewed By: rnk

Subscribers: xbolva00, lebedev.ri, lkail, merge_guards_bot, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71993

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