On 64-bit target, when doing i64 BR_CC where one of the comparison operands is a
constant zero, try to fold the compare and BPcc into a BPr instruction.
For all integers, EQ and NE comparison are available, additionally for signed
integers, GT, GE, LT, and LE is also available.
Reviewed By: arsenm
Differential Revision: https://reviews.llvm.org/D142461
c30c291887