[X86] Use APInt instead of SmallBitVector for tracking undef elements in...

[X86] Use APInt instead of SmallBitVector for tracking undef elements in constant pool shuffle decoding

Summary:
SmallBitVector uses a malloc for more than 58 bits on a 64-bit target and more than 27 bits on a 32-bit target. Some of the vector types we deal with here use more than those number of elements and therefore cause a malloc.

APInt on the other hand supports up to 64 bits without a malloc. That's the maximum number of bits we need here so we can avoid a malloc for all cases by using APInt. This will incur a minor increase in stack usage due to APInt storing the bit count separately from the data bits unlike SmallBitVector, but that should be ok.

Reviewers: RKSimon

Reviewed By: RKSimon

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D30386

llvm-svn: 296352