[APInt] Add APInt::insertBits() method to insert an APInt into a larger APInt

[APInt] Add APInt::insertBits() method to insert an APInt into a larger APInt

We currently have to insert bits via a temporary variable of the same size as the target with various shift/mask stages, resulting in further temporary variables, all of which require the allocation of memory for large APInts (MaskSizeInBits > 64).

This is another of the compile time issues identified in PR32037 (see also D30265).

This patch adds the APInt::insertBits() helper method which avoids the temporary memory allocation and masks/inserts the raw bits directly into the target.

Differential Revision: https://reviews.llvm.org/D30780

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@297458 91177308-0d34-0410-b5e6-96231b3b80d8