[DAG] Calculate the number of sign bits for constant BUILD_VECTOR directly.

For constant BUILD_VECTORs the operands need to be legal types. This can mean
that when the number of sign bits is calculated it may look that the entire
constant and inefficiently produce less sign bits than it could. For example i8
vectors could use i32 elements, for which 0x000000ff would be incorrectly
limited to 1 sign bit as the original value has 24 sign bits. This makes it
look at the constant directly, truncated to the correct type for the element so
that it can correctly return 8.

Differential Revision: https://reviews.llvm.org/D149956