[InstCombine] Avoid combining the bitcast of a var that is used as both...

[InstCombine] Avoid combining the bitcast of a var that is used as both address and result of load instructions

This patch fixes https://llvm.org/bugs/show_bug.cgi?id=27703.

If there is a sequence of one or more load instructions, each loaded value is used as address of later load instruction, bitcast is necessary to change the value type, don't optimize it.

llvm-svn: 270135