[APInt] Optimize APInt creation from uint64_t

Summary:
This patch moves the clearUnusedBits calls into the two different initialization paths for APInt from a uint64_t. This allows the compiler to better optimize the clearing of the unused bits for the single word case. And it puts the clearing for the multi word case into the initSlowCase function to save code. In the common case of initializing with 0 this allows the clearing to be completely optimized out for the single word case.

On my local x86 build this is showing a ~45kb reduction in the size of the opt binary.

Reviewers: RKSimon, hans, majnemer, davide, MatzeB

Reviewed By: hans

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D30486

llvm-svn: 296677