[InstCombine] Fix big-endian miscompile of (bitcast (zext/trunc (bitcast)))

Summary:
optimizeVectorResize is rewriting patterns like:
  %1 = bitcast vector %src to integer
  %2 = trunc/zext %1
  %dst = bitcast %2 to vector

Since bitcasting between integer an vector types gives
different integer values depending on endianness, we need
to take endianness into account. As it happens the old
implementation only produced the correct result for little
endian targets.

Fixes: https://bugs.llvm.org/show_bug.cgi?id=44178

Reviewers: spatel, lattner, lebedev.ri

Reviewed By: spatel, lebedev.ri

Subscribers: lebedev.ri, hiraditya, uabelho, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70844