[Support] Add some helpers to generate bitmasks.

Frequently you you want a bitmask consisting of a specified
number of 1s, either at the beginning or end of a word.

The naive way to do this is to write

template<typename T>
T leadingBitMask(unsigned N) {
  return (T(1) << N) - 1;
}

but using this function you cannot produce a word with every
bit set to 1 (i.e. leadingBitMask<uint8_t>(8)) because left
shift is undefined when N is greater than or equal to the
number of bits in the word.

This patch provides an efficient, branch-free implementation
that works for all values of N in [0, CHAR_BIT*sizeof(T)]

Differential Revision: https://reviews.llvm.org/D32212

llvm-svn: 300710